11 December 2011

Walking Around Your Geophysics Results

Wouldn't it be really cool if you could walk around your geophysics results, with an image on a screen with your location marked. You could get a good idea of the layout of things on the ground, or easily conduct a guided tour. Well you can! This rather lengthy blog post with describe two methods of doing so, each with their own benefits. This post is a follow on from a previous post, where I describe how to display geophysics results on Google Earth. This post will assume you have already got that far, and you have a geophysics overlay on Google Earth that you wish to walk around.

Google Maps

Unfortunately, the mobile versions of Google Earth and Google Maps do not currently (at the time of writing) support image overlays, so you can't just put a kmz on your phone and view it. What they can do is view vector graphics, but not directly from a file. Say you wanted to display some vector graphics (i.e. lines or placemarks), first save them to a kml file. Now go to Google Maps (on your computer, not your mobile device), where you will see a button called 'My places'. Clicking that will show you a list of vector files you have uploaded to Google. You can upload a new one by 'CRATE MAP' button. Give it a name, and select whether or not you want it publicly available, then click the import link and upload your kml file. Finally press the 'DONE' button.

Now onto your mobile device. Turn the GPS on if necessary, and load up Google Maps. Press the layers button at the top. One of the options will be 'My Maps'. Selecting this should bring up a list of the files you have uploaded. Selecting one will display it on Google Maps. If only this worked with image overlays, that would be nice and easy. You could of course draw some points, lines and polygons over features on your geophysics results and use that, but that would be cheating. We want a real image overlay! So here we go, two methods of doing just that.

Method 1 : With A Laptop

The first method will use a laptop, running Google Earth. I am assuming that the Windows operating system is used. Apologies to those that don't use it. Using a laptop means displaying the image overlays is not a problem, but causes us two other problems.

Firstly, Google Earth needs the internet to display it's own imagery. If you somehow have access to WiFi in the middle of a field, well that's nice for you, but most likely you won't. The easiest way to deal with this is tethering to a mobile device. Android devices have the ability to set themselves up as a WiFi hotspot, transferring the data over the phone network. Beware though, if you get a phone free, or cheap, as part of a package from a carrier, it is more than likely that they will have disabled tethering, because they don't like people actually using devices how they want. If you buy a SIM free phone yourself and get a SIM from a carrier separately, you should be alright. Make sure your contract has a monthly data allowance, or you will end up paying a lot of money.

Secondly, laptops don't have a GPS in them, but we can get around that as well. You can buy stand alone GPS units that can connect to a computer via USB. They are quite cheap. Personally, I have a QStarz BT-818XT, which works fine with what I am describing. You may have noticed that Google Earth has some options for dealing with GPS, but personally I have not much luck with it. I tend to use a program called EarthBridge instead. So here are the steps for using it.

1) Plug your GPS into your laptop and turn it on.
2) Check which COM port that your GPS device appears as. You might need to load a driver for it for this to work, which is dependant on the device used. You can see which devices are linked to which COM port in the Device Manager in Windows.
3) Load Google Earth.
4) Load EarthBridge.
5) Go to the 'Preferences' tab in EarthBridge. Change the 'COM Port' to the one observed in the Windows device manager. It is usual for the other connection settings to be 9600/8/None/1/None, but check with the documentation for your GPS to see what the setting should be if you have problems. Set a suitable folder for 'Save KML files to:'. You will need this for later.
6) Press the 'Connect to GPS Device' Button at the bottom. If all is well, you should see some NMEA output and satellite information in the 'GPS Status' tab.
7) Now we want to configure how things appear on Google Earth, so go to the 'KML Output Settings' tab. UNTICK the checkboxes marked 'Use Altitude Data If Available', 'Show the track information' and 'Fly To Placemark on Update'. Get rid of the placemark title and disable 'Show Speed'. Now you can press the 'Start' button at the bottom.
8) Go to Google Earth and in the 'Add' menu, select 'Network Link'. Browse to the directory you noted in point 5, and select the earthbridge-data.kml file. You should now hopefully see a dot representing your position appear on Google Earth, woohoo! The dot is a bit chunky, it could do with being a cross-hair, but it will do.

On the plus side, you have all the functionality of the full version of Google Earth, with turning layers on and off, and scrolling around however much you wish. On the minus side, carrying around a laptop with a GPS dangling from it is clunky, plus it is never easy to see a laptop screen outdoors.

Method 2 : With a Mobile Device

If we don't like lugging a laptop around a field, you can use a more portable device, such as a smartphone or tablet. Here, I will be assuming that an android device is used, but a method similar to this may work with other mobile operating systems. Unlike with the laptop, we will have no problem with the GPS or internet access, but we will have problems displaying the image in the first place. As mentioned above, the mobile versions of Google Earth/Maps don't currently support image overlays, oh woe and thrice woe. There is a way to do it though, but it is a dark and convoluted path, not for the faint hearted. The rumours of animal sacrifice involved in these dark arts are entirely fictitious. Fortunately, all of the software involved is free.

1) You will need lots of software for this task. First of all download the OSGEO4W installer. Run it, select 'Advanced Install', then 'Install from Internet', then 'All Users' and select the directory into which you want all this software installed, remembering this for later. Use whatever 'Local Package Directory' it chooses, unless there is a problem, then tell it what kind of internet setup you have. You will see various categories of software that you can install.
2) From the Commandline_Utilities category, make sure that gdal and proj are selected. From the Desktop category, make sure that qgis is selected. Everything else needed should be automatically selected as dependencies of those. pressing the Next button will start the install.
3) Even more software, download OruxMapsDesktop. It doesn't have an installer since it is a java application, so just unpack it from its zip file and put the directory somewhere.
4) Yet more software. Install the OruxMaps application on your android mobile device. You can search for it in the android marketplace. It's free.
5) Now we have all the software we need we can begin. Start by creating a directory somewhere where you can put all of the files associated with this. I have used the directory E:\Data\gis\oruxtest, so if you see that directory in the following explanations, you will know what that means, and you can replace that with whatever directory you are using.
6) The area that you are viewing on your mobile device will be limited to a single image that you can save from Google Earth as a jpg, so you need to pick the area you are viewing carefully. This means no scrolling around a large area, and you can't turn layers on and off. Bear in mind that if the area you are viewing is too large, you wont see much resolution on the geophysics, and wont be able to make out the features very well. If the viewing area is too small, you will only have a tiny area to walk around. Don't forget that as well as using your mouse wheel to zoom in and out, you can get a finer control on the zoom by holding down the right mouse button and moving the mouse up and down. Once you have selected the are you wish to walk around, we need to mark it up for georeferencing.
7) You need to add 4 placemarks to the area you are viewing. It helps the next process if they are arranged in a square or rectangle, and it helps you if they are reasonably round numbers. They will also need to be near the corners of the area you are viewing. Right click on your placemarks and select Properties. So they don't get in the way too much, change the icon (button at top right) to a small circle and reduce the scale of the icon ('Style/Colour' tab) to 0.7. Big enough that you can see it on the resulting image, but not too big that it gets in the way. Reduce the scale of the label to 0.1, because we don't need it and don't want it getting in the way. Manually change the Latitude and Longitude so they are roughly in a square or rectangle around the area you are viewing. This is best explained by the example image below, and some real numbers associated with it. If you click on the image below, you will see the four dots, one in each corner. Their coordinates are as follows :

Top-left: 50.914000 Lat 0.022000 Long
Top-right: 50.914000 Lat 0.026000 Long
Bottom-left: 50.911500 Lat 0.022000 Long
Bottom-right: 50.911500 Lat 0.026000 Long

You can see that the numbers have been manually rounded, so as to make a rectangle. You will need to make a note of the numbers that you use for later.

8) Once you have the 4 georeferencing points and the display area perfect, you need to save an image of what you can see, just like in the image above. Go the the 'File' menu, then 'Save', then 'Save Image...', and save the file in the directory that you created in point 5. In this example, the file I have used is called bartest.jpg, so if you see that name later on, you know that you can replace it with your own filename.
9) Now we need to georeference the image. For this, you will need the coordinates that you noted in point 7. If you go to the windows start menu, you should see some of the new software you installed earlier, in the OSGEO4W menu. In that menu, you will find Quantum GIS. Run that now
10) We will need the georeferencer, so go to the 'Plugins' menu, then Georeferencer > Georeferencer.
11) We  need to load the image we have just exported from Google Earth. Click the leftmost icon at the top. If you hover the mouse pointer over it, it should say 'Open raster'. Click on this and select the file you have saved from Google Earth. It should appear on the screen in front of you, with the four small dots near each corner.
12) Click the Zoom In tool up the top. It looks like a magnifying glass with a plus underneath it. Click a couple of times on the dot in the top-left of your image, it should be a lot clearer now. Select the 'Add point' icon at the top. It looks like 3 dots. Aim the mouse pointer at the centre of the circle and click. You will be presented with a window that asks for X and Y coordinates. You will need the coordinates that you noted from point 7. Enter the coordinates relating to this point, with the Latitude going in Y and the Longitude in X. Then hit OK. You need to repeat this for the other three points, which can be reached with the 'Zoom Out' icon, which looks like a magnifying glass with a minus under it, and/or the 'Pan' icon, which looks like a hand. You should end up with something like the image below. Make sure the coordinates are typed in correctly, it is very easy to get them wrong and mess up this part of the process.

13) Once the coordinates for those four points have been entered, click the 'Transformation settings' icon up the top, which looks like a spanner. Set the 'Transformation type' to 'Helmert', the 'Resampling method' to 'Cubic' and the 'Target SRS' to 'EPSG:4326'. The 'Output raster' should be the same filename as the file that you selected to begin with, but with an extension of .tif, so for example 'E:\Data\gis\oruxtest\bartest.jpg' is output as  'E:\Data\gis\oruxtest\bartest.tif'. Once this is all filled in, click OK. An example of how the screen should filled out is given below :

14) Click the 'Start georeferencing' button, which looks like a green play button. This should create a georeferenced image in your directory. This image is a TIF file, which is a standard type of image file, but embedded within this is information as to the geographic location of the image. Embedding this information turns this file into something called a GeoTIF. Find your working folder in windows and open the image. You will probably see that the image has been rotated slightly, with the surrounding area filled in with black. If you find that the image has been rotated a lot, or it has been stretched in one direction, then you have probably entered some of the coordinates wrong, and must start again. If all is well, then we can progress to the next stage. The next stage doesn't seem to like GeoTIFs however, so we must convert it. I wont go into the details of the whys or wherefores too much, I will just tell you what you need to do.
15) To make this easier for the second time around, and to save typing, we are going to create a batch file. In your working directory, create a text file, and put in the following four lines :

c:\osgeo4w\bin\gdal_translate -co "TFW=YES" %1.tif intermediate.tif
c:\osgeo4w\bin\gdal_translate -of PNG %1.tif %1.png
rename intermediate.tfw %1.pgw
del intermediate.tif

If the OSGEO4W software was installed in point 1 was installed anywhere else that c:\osgeo4w, when you will need to change these to the correct location. Save the file, then rename it to 'orux_translate.bat'
16) A new piece of software now, the OSGEO4W shell, in the 'OSGEO4W' menu, there is another menu called 'OSGEO4W' followed by 'OSGeo4W Shell'. Run this.
17) This is like a normal Windows shell, except a few other things are set up for us. We now need to get to our working directory. In my case, that is 'E:\data\gis\oruxtest', which means I should type :

cd \data\gis\oruxtest

18) We can now use the batch file we have created to convert our GeoTIF image into something that the next piece of software can use ("More?", I hear you cry, "You want more?"). Type :

orux_translate bartest

Of course you need to replace bartest with the name of your file, without the extension at the end.
19) Almost there, don't give up now! Now we need to run the OruxMapsDesktop software. In the directory into which you installed the software in point 3, you will find 'OruxMapsDesktop.bat'. Run that.
20) The resulting screen should be filled out as per the image below. Ignore any warnings it brings up along the way. Click 'Calibration file' and select the file in your working directory ending '.pgw'. Click 'Image file' and select the file ending in '.png'. For the DATUM drop down box, select 'WGS 1984: Global Definition'. It is near the bottom of the list.For the PROJECTION drop down box, select 'LATITUDE/LONGITUDE'. In the box next to the 'Destiny Directory' button (the original language for this software is not English, can you tell), enter your working directory. Select 'png format' for the output. Click the 'Create Map' button.

21) If you now look in your working folder, you will see a new directory has appeared which contains an 'xml' and a 'db' file. You need to copy the whole directory, not just the files, to your mobile device. Turn on your mobile device and connect it to your computer with the USB charging cable. A 'Connect to PC' dialog will appear on your mobile device. Select 'Disk Drive'. You should not be able to treat your phone like any other USB mass storage device. Copy the directory you have just created into a directory on your phone called \oruxmaps\mapfiles. Now 'Safely Remove Hardware', just like you would with any normal USB mass storage device.
22) Go out into the field you want to wander about in. Make sure GPS is activated on your mobile device, and then load the OruxMaps App.Select Map offline, press 'Reset Map Sources' if your map is not there, then select your map. You should see something like in the image below. Press the wiggly line up the top and select 'Start GPS', and away you go, you can wander about your geophysics on your phone. Huzzah! The default cursor of the giant arrow is a bit of a blunt instrument for walking around in such a manner. You can select a better cursor in Settings > User Interface > Cursor > Cursor Icon. 'neodraig2' seems to be the best one there currently.

On the plus side you can walk around your geophysics using a genuinely portable device with a screen that works well outside. On the downside, you are limited to a single Google Earth image to walk around, and you can't do anything with layers. Plus, it's a hideously complicated process to do each image. If you thought it was complex to reproduce what I have done, imagine what it was like working out how to do it in the first place!


You might think that this is a good way to position trenches, but you would be wrong. Whilst survey grade GPS systems will get you to within a cm of where you want. consumer grade GPS's will get you within 6-8 metres of where you want, and there doesn't seem to be much available between the two extremes. There is also the Google Earth error. The imagery on Google Earth is not always exactly placed, so you make get another couple of metres error there. So as I mentioned at the start of the post, this is for walking around the geophysics and getting a feel for the place, but you can't rely on it being precise.

Hibernation Time

Well it's getting a bit chilly out there now, too chilly for geophysics, or rather too chilly for me doing geophysics, so this blog will be going quiet over the winter. Hopefully I will have some new and interesting results for you in the spring.

21 November 2011

Fourier Transforms

This post is going to get fairly technical, but the results are worth it, so please bear with me. I will start off with some technical terms, which you don't need to remember. For the uninitiated, Fourier Transforms are a pretty hairy set of mathematical functions that deal with frequency. In their purest form, they deal with the frequencies inherent in a mathematical function. This is of no use to us though, when dealing with digital data, that can't be described purely with a mathematical function. We need a special type of Fourier Transform, known as the Discrete Fourier Transform. The discrete here refers to the individual pieces of digital data that we are working with. The most common implementation of this, used in computing, is the Fast Fourier Transform, or FFT as I will refer to it from here on.

Still with me?

What exactly does all this fancy maths do then? It will analyse a set of data, and turn it into a set of frequencies. You can then apply an FFT Inverse to this set of frequencies to get back the original piece of music. What use is all that then. Let me give you an example  The application for all of this most people will be familiar with is sound, specifically music. Imagine you have a piece of music, say 10 seconds long, with for example, a base guitar and some cymbals. You then apply an FFT filter to it, which will transform it from the time domain, to the frequency domain. The data you will see after transformation will show some low frequencies, which would be the base guitar, and some high frequencies, which would be the symbols. You could just apply the inverse transform, to take it back from the frequency domain to the time domain, and reconstruct you piece of music, but (and here is the clever bit) you could remove the high frequencies from the transformed data first, before the inverse transform. You would be left with a piece of music where the base guitar was unaffected, but the cymbals would be almost inaudible. Pretty good huh? If you have a graphic equaliser on your stereo, you will be familiar with the kind of effect I am talking about.

Still with me? Well done.

So what does this have to do with geophysics? In two words, plough lines. Ploughing can leave a lot of streaky lines across your nice geophysics plot. Take the image below as an example, fresh from my last blog post. If you look closely, you can see a lot of plough lines, heading ENE-WSW, cluttering up the place like unwanted junk mail. So what can we do about it? If you imagine our piece of music as being one dimensional, with the dimension being time, you can imagine the image below having two dimensions, X and Y. Specifically, the piece of music is in the 'time domain' and the image is in the 'spatial domain', and we can use even more hairy maths on it, a two dimensional FFT. If you imagine the plough lines as waves crossing the image, we can get a specific frequency for it, and remove it, thereby removing the plough lines from the image.

Here are the steps you will need to take to accomplish this, which fortunately does not require you to do any of the maths yourself, and uses free software. You can click on these on the blog to view bigger versions, which you will probably need to do to see what I am talking about.

1) Firstly, you will need to download piece of free image processing software, called GIMP. Lovely name huh? It's actually pretty powerful.

2) Next you will need to download the FFT filter for GIMP, installing it as described for your system.

3) Now open the image you want to work on, and use Filters > Generic > FFT Forward. You will be left with a very strange image, as shown below.

4) The next stage is quite difficult to explain, I will do my best. The image above, which is somewhat zoomed in, shows the frequencies in the transformed image. It is now in the frequency domain. You will see a cross, radiating from the centre of the image. The centre is where the high frequencies are, going to lower frequencies towards the edge. You will need to zoom in to find what you are looking for, which is a pair of lines of higher readings amongst the frequencies. Due to the nature of the filter, you will get this somewhat mirrored effect. This is the hardest part, and may require some trial end error on your part.

5) Once you have found the relevant frequencies, you will need to surround them both with the Free Select Tool (looks like a lasso), which I have done in the image above. Try and enclose as small a space as possible around the frequencies you wish to remove, otherwise your image may suffer for it. You can do one at a time if you wish.

6) Use the Blur Tool (looks like a drop of water), within the selected area. You don't have to worry about going over the edge, as the selection restricts any changes you make to within the selection. Make sure you give it a good going over with the blur tool.

7) Once both areas have been blurred, remove any selections using Select > None.

8) Now we are ready to go back to the spatial domain. Use Filters > Generic > FFT Inverse.

9) You should hopefully now have your geophysics image with most of the plough lines removed. If you haven't, there is always Undo, and you can try again.

10) In some areas where the plough lines did not originally encroach, you may find that the filter has introduced some artefacts, that look like the original plough lines, which isn't very helpful. These areas can be restored by copying and pasting the parts of the original image that you wish to revert to their original state.

Well, here is the resulting image. Open both the original and filtered images and compare them. The effect is quite striking. There are a few small pieces of the ploughing left, but it has mostly been removed.

20 November 2011

Latest Results: Alfoldean

My final major survey of the year is on the Roman mansio at Alfoldean, with the Horsham group. The site is on Stane Street as it crosses the Western Rother, which the modern A29 follows. After Winbolt (SAC 64, p.81), Time Team had excavated here, re-excavating much of what Winbolt had already done. Time Team had surveyed a strip along the road to the west, but not to any great distance into the field. This survey went right across the field to the west, but not as far south as Time Team. We may return to do some more.

The mansio enclosure ditches are clear in the top-right of the image as a dark strip. This is also clear on the ground as a wide dip in the field. There is a break in this dip on the western side, and this is consistent with what looks like a track leading out, before disappearing briefly, the track seems to split, heading in a curve to the south, and in a straight line to the south-west before stopping at a perpendicular ditch, which seems to be part of another enclosure. It is in this area that a large spread of the sort of tiles that suggest a bath house are visible on the surface of the field. There are features within this enclosure, and one of them is particularly strong. Is this the location of the stoke hole for the bath house?

Well now it is getting towards winter, and my feet definitely need a rest, as I have developed Plantar Fasciitis due to excessive geophysics. Expect to see some more geophys results in the spring folks. Take care all.

25 October 2011

Latest Results: Steyning

Working on the theory that large scale Roman occupation is common where Roman roads cross large rivers, I worked with the Independent Historical Research Group to try to find such settlement where the Sussex Greensand Way crosses the River Adur. Initially, to the west of the river, the results were promising. The magnetometer showed some Roman settlement next to the road, but the road was 80m north of where it was supposed to be. The red line shows Ivan Margary's course for the road, with the corrected line shown in green. The strange course for the road may have something to do with the steep slope that the road climbed up.

We were looking for something more substantial though, and there was plenty of pottery on the other side of the river, where the ground was not as steep. Unfortunately, due to very heavy contamination with metal, the area on the east side of the river was a complete mess on the results, so the Roman remains here will remain a mystery for a while longer. The plot is below anyway. Eeewwwwww! You can see the full report here.

13 October 2011

Magnetometry Junk

Interpreting geophysics results is like sorting wheat from chaff, and you have to be able to recognise a lot of different chaff, otherwise you end up digging it up. A while ago, I had the dubious fortune of surveying an area that contained pretty much all the types of chaff you are likely to come across in a magnetometry survey. The site is in a valley that runs roughly north-south down the centre of the image.

A is the actual archaeology we are looking for. It's a small Roman period iron bloomery that was excavated by Henry Cleere in the 1960's*.

B is a water pipe. An off-shoot of this leads to a water trough. These are easily recognised by their stripey character and high readings.

C is an electricity cable, leading to the house at the northern end.

Covering the survey are are land drains, such as at D. They are generally ceramic pipe, and help drain the land of excess water. Modern land drains may be plastic and less easy to spot.

Just to the right of E is a small feature, half black and half white. This is known as a dipole, and is generally indicative of modern metal junk. You tend to get these more on arable land as bits break off tractors as they work.

F is a plastic pipe dug into the centre of the valley to take the small stream underground. Whilst the magnetometer does not pick up the plastic, it does pick up the cut into the underlying geology in which the pipe sits.

Just in the corner at G, we surveyed close to the metal fence. Fences tend to show up as negative readings.

The two blobs at H are actually down to the landowners landrover pulling up as I neared the end of the survey line. Be sure to make a note of such things as you survey, otherwise you might mistake them for features

*Cleere, H.  The Romano-British industrial site of Bardown, Wadhurst. Sussex Archaeological Society occasional papers no. 1

12 October 2011

Writing a Geophysics Report

Compared to excavation reports, geophysics reports are really quite simple, and fairly quick to write. If you were to look at standard grey literature geophysics reports, it may seem otherwise, for two reasons. Firstly, commercial units like to pad out reports with unnecessary waffle, to make the customer feel like they are getting their money's worth. Much of this will be copied and pasted between reports with little alteration. Secondly, geophysics reports may also include a wider desk-based evaluation of the surrounding area as part of the contracted work, which may not be necessary otherwise. So what is the minimum required of a geophysics report? Here are a few things to include :

  1. A bit of background about what is already known about the archaeology of the site, and why the survey is being undertaken.
  2. A short statement about the equipment used and how it is being used.
  3. Notes on the positioning of the survey grids, in such a form that those grids can be re-established by someone else.
  4. A plot of the results.
  5. Interpretation of those results.
After you have finished your report, what do you do with it? Giving a copy to the landowner is always a nice thing to do, as well as the county archaeologist (if you are in the UK) or equivalent in your country. There are several options if you want to publish your results in some way. Local archaeological societies will be quite happy to publish surveys in their area in their newsletter or journal. Some of the larger county or national journal may not be interested in pure geophysics in their own right, and will probably charge you for the privilege if they do. The other option is to publish on-line. I use a site called scribd, which allows sharing of documents on the web for anyone to download. You can see a list of my already uploaded reports here, which will give you some examples of geophysics reports if you intend to write one yourself.

17 September 2011

Annoying Geology 2

Look at the magnetometer image on the left. There is some pretty obvious archaeology there, a couple of tracks and a small enclosure, but what are those large blobs? They must be archaeology too right? I certainly thought so, I even flagged them as archaeology in the report. Similarly with the image on the right, showing a track next to these features.

Actually, this is more annoying geology. This time, going under the name of Gley. Gley is often formed geologically adjacent to streams or other water courses. The example to the left is next to a stream, and the right next to the floodplain of a river. You don't want to dig that stuff up, very sticky.

12 September 2011

Which way do I walk?

When doing a magnetometer survey, questions arise of which way should you be walking across the grid, and which corner do you start in? Here are some things to consider when making that choice. They may well conflict with each other, so it is up to you to decide how much weight to give each of them.

  • If there is a slope in your field, you will want to walk along it rather than up and down the hill. This not only saves your legs, but will help to avoid stagger in the results.
  • A funny shaped field can affect your choice, and the correct choice will mean less messing about with dummy readings. For example, if you have a rectangular field with many partial grids along one edge, then you will want to walk parallel to that long edge and start in the corner away from it. That way, you will only need to press 'end grid'. Another example, this time with a field which has a straight edge, and opposite to it a very curving edge. Again you will want to start in a corner away from the curving edge, but instead of walking parallel to it, which would result in problems when you hit the edge, walk towards it.  Magnetometers have a feature where you can finish the line and automatically enter dummy readings to the same point on the next line. Starting opposite the curved edge avoids you entering dummy readings at the start of a line. Counting marks on the string to start a line sucks.
  • If you are surveying a field, its present condition may have an effect on which way you walk. It will be easier on the legs to walk along lines of ploughing or stubble than across them. 
  • The archaeology itself can have an impact on which way you walk. If, for example, you are expecting a Roman road crossing your survey area, you will want to walk perpendicular to it for two reasons. Firstly, you will have a greater resolution of readings across the feature, and are more likely to pick it up. Secondly, use of the destripe filter may hide features that are parallel to the way you walk.
  • Converselywise, if you have a known utility pipe across your survey area, walking parallel to it will help stop the extreme readings from adversely affecting the destripe filter across the rest of the grid.
  • The hardware itself may play a part. For example, with the Bartington GRAD601-2, the data collection procedure expects the left sensor to always be on the edge of the grid with the first line that you survey, so picking a direction to walk will restrict you to two out of four corners to start in.
  • Hardware again. Most machines in use today are fluxgate gradiometers, and produce striping in the results. With the Bartington at least, the balancing procedure results in less striping heading north-south than east-west. Filters can deal with striping though, so you only really need to take this into account if you have a flat square field and no idea about the archaeology underneath. 

05 September 2011

Earth Resistance Replacement?

I've been pondering the nature of earth resistance in archaeology, mostly because it is so darn slow. There has to be a quicker way of getting this sort of data. I'm sure you all know the standard way of doing things, with a twin-probe array. You also know that the main thing it reacts to is a change in the moisture in the ground. There are other ways that you can measure ground moisture, however, without sticking metal probes in it. I must admit that I know little about these methods, and hence I am looking for comments on their possible effectiveness.

Electrical Conductivity

Conductivity is already established as an archaeological tool, albeit a little used one (at least in the UK). It works by inducing a magnetic field, similar in a way to a metal detector, though not quite. As you can see (with the Geonics instruments), it has a similar collection method to magnetometers, which is good. Unfortunately, the effectiveness compared to traditional resistivity has been questioned, with the results apparently being a lot less clear.

Thermal Imaging

As everyone knows, water is very good at holding heat, but can this be exploited for archaeological purposes? Thermal imaging cameras are commercially available, for example, from FLIR. They can be cheap to astoundingly expensive, depending on the sensitivity and resolution of the instrument. The question is, does any temperature difference under the surface have a measurable effect on the surface, where the camera is looking? If so, it would mean a very quick means of collection, perhaps with the aid of a small tethered balloon to dangle the camera from, or a UAV, assuming it can carry the weight. Most likely though, it wont be as effective in getting the depth needed.

Microwave Radiometry

Jumping past the other end of the visible spectrum, there is another means of measuring ground moisture, with a microwave radiometer. I was first alerted to the possibilities by this article. Their general use is for meteorology, and deployed in satellites, but you can buy smaller versions. Is it possible to point one of these at the ground and measure the moisture? What would be the depth penetration? How would it be affected by plants? Again it may suffer from the same limitations as thermal imaging.

If anyone knows more about these the science involved, then please post a comment. It's all hardware to me, and I'm a software guy.

01 September 2011

Overlaying Geophysics Results on Google Earth

I've been asked before (Hi Lisa) how I overlay geophysics results on Google Earth. My method requires the use of a total station, which though not cheap, is considerably less expensive than an RTK GNSS setup. You can pick up a decent 2 person setup (i.e. non-robotic) on ebay for about £3000.

So as an example, let's take this field :

Step 1) Set up your total station in a position where you can see all of the features you wish to record (see below). This may be difficult on undulating terrain, so think before you set up.

Step 2) Optionally, you can use the total station to lay out any grids rather than using tapes, which is more accurate over a large area, especially if it is hilly. You need to use an arbitrary grid for this, so set your x,y coordinates to be, for example, 500,500, and set the horizontal angle to a suitable value whilst pointing the total station in the direction of the grids. On my total station at least (Topcon), 0 corresponds to grids north, 90 is grid east, 180 is grid south and 270 is grid west. Once this is done, you can send out your staff bearer with a walky-talky and give them directions to go so many metres/centimetres (grid) north, south, east or west until they hit the right spot to mark a corner of a survey grid.

Step 3) If you have not set yourself up in an arbitrary grid, do so now. You will need to record three things :

Step 3a) Record the corners of the survey area, which is only 4 if you have a square or rectangular survey.

Step 3b) Record two resection points. These are points which you can accurately describe, on features that are unlikely to move any time soon, for example, telegraph poles or gateposts. An example description would be 'Centre of south face of western gatepost of gate north edge of field'. This is a point which someone should be able to find to within a few centimetres. When you come back to the field and want to re-establish the arbitrary grid, you enter the coordinates you have recorded for the resection points, and re-survey them. The total station can then work out where it is in relation to the original grid, setting its own position accordingly.

Step 3c) Record several features that you can see on Google Earth. The edge of the field is good. Several point recorded along a fence will give you a nice line to match up with the aerial photograph. You will need to record more points if the fence turns or curves. Recording where the orientation changes is a good idea. Trees at the edge of a field can obscure the fence on the aerials, you will generally need at least two sides of the field to have a visible fence for this process to work well.

Step 4) Do your geophysics survey

Step 5) Download the data from the total station and export as a dxf file. Load this into a CAD program. If you don't have AutoCAD and don't want to spend a lot of money, something like QCad is a cheap option. Join the dots of the survey area, and any features like the edge of the field. If your survey area is less than 100x100 metres, you will need to add a two dimensional scale of at least that size, anywhere you wish on the image. You should end up with something like this :

Step 6) Save the CAD image in a lossless raster file format that supports transparency, I tend to use PNG. White lines tend to show up on Google Earth better than black lines, so make the background black. Load this image into a decent paint program, and make the black background transparent. The method of doing so will differ between paint programs, so you will need to consult the documentation.

Step 7) Add the resulting image to Google Earth as an image overlay. Use the measure tool on the two dimensional scale, or your survey area, to get the overlay to the correct scale. Rotate and shift the image so that the features you have surveyed, such as the field edge, match up with the aerial photo. Remember that the lines drawn on are a guide. It's the points that really matter. If there is a curve in the field boundary, and you have not taken enough points, then the lines between the points will not exactly match up to the aerial. You should end up with something like this. Note how the white lines match up to the features surveyed :

Step 8) There will now be a nice box into which you can drop your geophysics image. Don't forget that if there are any dummy readings or missing grids in your survey, then you will need to make that area transparent in the same way as the grid image. If you are using a standard grey scale for your geophysics plot, it is best not to make your dummy readings black or white, otherwise that may interfere with the transparency. Make dummy readings a completely different colour, such as red. And finally you are done. As you can see, the field has been completely trashed during the building of the adjacent reservoir, which is a shame.

A Note on Google Earth Imaging

You may be thinking, why not just take four readings of your survey area with an RTK GNSS system and save yourself all the hassle. To an extent, you would be right. Apart from the cost, there is a problem with the Google Earth imagery itself, in that it is usually not exactly placed. It may be only a couple of metres out, but that may be enough to confuse things, meaning that your survey will not be correct in relation to the image.

Converselywise, if you think you can get decent geographic coordinates from your total station measurements using Google Earth, you can expect a similar error going the other way.

If you have an RTK setup, don't forget that they generally record in a coordinate system that is tied to a continent, for example ETRS89, whereas Google Earth works with the world average, WGS84. The difference may only be half a metre, but it increases every year.

29 August 2011

Latest Results: Pond Field at Barcombe

As part of the Culver Project, I've been working with Rob Wallace to track his Roman road across the fields. We surveyed a new field this bank holiday weekend, and the results are great! The road takes a strange curve, and there is settlement on both sides of it, partly obscured by a metal pipe. The other annoying thing about this metal pipe is that it carries on over the stream to the north, where my nice line of Roman iron-workings has turned into a modern metal pipe with some sections removed. I imagine this was a water pipe that once supplied a trough for animals, as it suddenly stop in the field to the north.

Rob has already dug up a section of the road in this field, and found burning and evidence of industry on site, which is why the ditch features show so clearly. I'm sure Rob will be excavating some more of this at some point, so if you are interested in getting involved, contact him at the Culver Project. Many thanks to Rob, John and my long suffering wife Merryn for helping out this weekend.

26 August 2011

Annoying Geology 1

So there you are, doing a nice little resistivity survey on the chalk hills, and you get this :

Wow, you think. Look at all those lovely features. There must be a lot of big pits there, and so you dig them up... and wish you hadn't. Even as you excavate them, you find what looks like an obvious cut and fill in the chalk, but it is only a ruse, for this is a particular form of geology called clay-with-flints. At one time, there used to be a layer of tertiary geology on top of all that chalk, mostly sandy or clay, for example, The Reading Beds. Most of this has been washed away over time, but some layers are still intact. In other places, these tertiary layers, being acidic, have eaten down into the chalk below, creating pit like features filled with clay and flints, hence the name.

This is of course intensely annoying when you are trying to find archaeology, because these features show up particularly strongly compared to any weaker looking archaeology next to it. This cloud does have a silver lining though. You wont find any features with a magnetometer on chalk, as it can't tell the difference between the soil and the bedrock, but if there is clay-with-flints in the area, the clay in the soil will help show up those cut features. You just have to spot them amongst all of the geology.

I have learnt a fair amount about the local geology from doing geophysics, and I will be sharing some of the quirks in this blog.

25 August 2011

Iron-Age Enclosure?

Let's start the blog off with a pretty picture. I did this survey with the Independent Historical Research Group a while back, and it's been one of my favourite surveys, despite it not being Roman. It was initially spotted by someone from the Hastings group as a crop mark on Google Earth, and as IHRG were already working next door at Bardown, they decided to get permission from the landowner. My initial thoughts from the aerial photographs were that we might have a henge, but that doesn't look like that is the case now we have the geophysics results.

As you can see from the magnetometer survey results, there is a double ditched enclosure, but not perfectly circular as you would expect a henge to be. There is a single entrance to the east through both ditch circuits, whereas you would normally expect two from a henge. The ditch circuits disappear to the south and north as the enclosure is built on an east-west ridge, burying these sections under colluvium. The nice strong readings from the ditches are due to charcoal, which was discovered in the fill using an auger. There weren't any finds on the surface, and no coins found with metal detectors, so probably not Roman, so we are going with Iron-age until it is dug. You can see the full survey report here.

The blog begins

I've been doing archaeological geophysics for a long time now. I haven't had any formal training in it, I just picked it up as I went along. I have bought a TR systems resistivity meter (with tomography kit) and a Bartington GRAD601-2, and I borrow a total station. I'd like to share with you some of the prettier survey results I've had, along with some of the things I have learned about archaeological geophysics over the years, to help those who are just starting to get into the field (pun intended). I will also, of course, be talking about the geophysics software I wrote, Snuffler.