Surveying for Archaeological Geophysicists

Something that archaeological geophysicists have to grapple with constantly is location. It's no good having a pretty geophysics image if you can't put it on a map or tell someone where to dig.   Not only do you have to set out your survey grids, but you have to record where they are. The whole process can be time consuming, taking away from your actual geophysics survey time, so it is best kept to a minimum if at all possible.

Tapes

Tapes are where it all started. Old hands at geophysics will remember marking out 20 metre grids off of a baseline using two tapes at 20m and 28.28m (yes, Pythagoras was good for something after all). Anyone doing this over any great distance will notice that their lines of grid markers start to curve off into the distance. It's even worse if you are surveying on a hilly area. Even then, how do you get your survey onto a map? Certain points in your gridded area can be measured to known markers in the landscape, at least two per point, so that the grid can be re-established. All in all, it's far from ideal, but fortunately, technology is here to save the day.

Total Stations

Total stations are the next step up. Like tapes, they have no idea where they are in the world besides what you tell them, so you will be using arbitrary grids. They do have the advantage of being quicker and more accurate over long distances. Grids can be set out by setting an arbitrary position and direction for the total station, then communication with the person holding the prism pole until they are in the correct position for the next grid marker. This part is quicker if you have a robotic total station, which can be used by a single person, but is more expensive. A previous grid can be re-established with resection points. These are two or more points in the landscape which you can describe with great accuracy, e.g. "NE corner of W gatepost of gate in SW corner of field". Alternatively, survey pegs can be placed somewhere where they are unlikely to be removed to act as resection point. When returning to re-establish the grid, you can then place your total station wherever you want, then measure your resection points, telling the total station their grid locations, as previously surveyed, and the total station will calculate where it is within your original survey area. If you pick the right resection points, you can re-establish your grid with an accuracy of 1cm. It is best to keep these points a long way away from each other, to reduce rotational error. As for getting your geophysics on a map using a total station, I have discussed this in a previous post.

When looking to buy a total station, there are a number of things to look for. The first is the accuracy. Each total station will be rated as accurate (for rotational angle) to a number of arc seconds, so 1" is better than 3" is better than 5". In addition to this, some basic models will only deal with X and Y coordinates, so if you are interested in height information (Z), you need to check for this. Robotic total stations (single user) will be more expensive than a normal total station (two user).

GNSS

Now to the real reason for this post. For some time, I have been looking at buying GNSS, to replace the total station I use currently, but there is a lot more to consider when buying one. The main advantage to using GNSS rather than a total station is that you will have absolute coordinates, rather than using an arbitrary grid. The subject can be a bit of a technical minefield, so I wanted to share something of what I have found so far. The field is changing rapidly, so no doubt this information will be out of date in quite a short amount of time. I will assume a basic understanding of how GPS works, to avoid this guide becoming too verbose.

GNSS (Global Navigation Satellite System) is the modern term for what most people understand as GPS. GPS is actually just one of four constellations of satellites, GPS (US), GLONASS (Russia), Galileo (EU) and COMPASS (China). The first two systems are mature, with a full compliment of satellites. The other two only have a few satellites in place, and can't reliably be used for positioning yet. These separate systems will give you different locations, your receiver may not use all of the constellations at once.

Many people will be familiar with the GPS functions in devices such as smartphones. These are consumer level devices, accurate to a few metres, which is far too inaccurate for our purposes. Ideally, we want 1cm accuracy. For that, we need survey grade equipment. To understand the benefits of survey grade equipment, we need to know a bit about the signals that the satellites use.

Each signal is composed of the code phase and the carrier phase. The code phase is a string of data, a pseudo random number (PRN) that allows the receiver to work out roughly where a particular satellite is. Because this string of data is quite long. you can only use this to work out where an individual satellite is in relation to you to within about 20m. Having a large number of satellites at your disposal will allow you to improve the accuracy to an extent, but only so far. Consumer grade devices only use the code phase, which is one of the reasons they have poor accuracy. The carrier phase is the carrier wave used to carry the data in the code phase. The wavelength is much shorter than the time it takes to transmit one PRN in the code phase, which makes it possible to use this for greater accuracy. Receivers that use the carrier phase are RTK (Real Time Kinematic), which at the moment, is only survey grade equipment.

Each constellation many have several signals, consumer grade equipment will currently only receive one of these. The more signals that your receiver can pick up, the more accurate it will be.

GPS : L1 (Consumer), L2 (code phase encrypted, carrier only), L2C (not yet active), L5 (not yet active)

GLONASS : L1 (Some consumer devices), L2

Galileo : E1, E5a, E5b

COMPASS : B1, B2, B3

To pick up the signals from satellites, your receiver will need an antenna. Consumer grade equipment tends to have quite poor patch antennas compared to the survey grade equipment. Also on survey grade equipment, the antenna is mounted above your head, so you aren't getting in the way of a weak signal, and will have a base plate, to stop multipath from the ground. Multipath is one form of inaccuracy that a receiver will need to deal with. A signal can bounce off of something before hitting the antenna, making the receiver think that the satellite is further away. As well as physically stopping multipath from the ground, survey grade equipment can reject multipath from other sources such as buildings using statistical processing.

Another source of error is distortions in the ionosphere and troposphere altering the speed of signals on the way down. The solution to this is corrections, which come in different forms. SBAS (Space Based Augmentation System) is the most basic, and is often available on consumer grade equipment. These are regional services, such as WAAS (US) and EGNOS (EU). The way it works is that base stations, which have known locations, will pick up the signals from satellites, and work out how the ionosphere and troposphere are distorting that signal. They will then broadcast those corrections to a satellite, which then broadcasts to your receiver, which applies those corrections to make your location more accurate. Unfortunately, the base stations are very widely spaced, so the corrections will not give us the 1cm accuracy we desire.

Traditionally, the way that survey grade equipment deals with the problem of corrections is to have a separate base station of its own set up nearby, on a known point. Corrections will then be sent by radio to the surveyor (rover), giving the desired 1cm accuracy. The longer the distance between the base station and the rover, the less accurate the corrections will be. The downside to this is having to find a known point, or waiting a long time to let the base station properly work out where it is. Also, if the base station is any distance away, you will need a second person to look after it, in case it gets stolen. The current way to deal with this problem is to have a closely spaced network of ground stations, which create a decent model of atmospheric distortions. The corrections are sent over the internet rather than via a satellite, with the receiver having a mobile network SIM card installed to pick them up. This is known as 'Network RTK'. Unlike SBAS, these corrections are not free, costing around £1000 a year in the UK, which has various network RTK services, all based on OS Net.

So... did you really need to know all that? Well yes, because if you are spending a large amount of money on a new piece of kit, you better understand exactly what it is doing, otherwise you might end up with a lemon that only receives GPS L1. There are a large number of options when buying survey grade equipment. Survey grade has traditionally been very expensive, but prices are starting to drop with increased competition. The satellite constellations are also improving, with GPS L2C and L5 due to be activated in 2016 and Galileo and COMPASS are reaching maturity, so I am going to be waiting a bit to see how receivers develop before making a purchase.

Coordinate Systems

Another thing you will need to understand if you are using GNSS is coordinate systems. I will discuss those relevant to the UK. There are two types of coordinate systems, geographic coordinates and projections. Geographic coordinates are based on a latitude and longitude, a number of degrees from a certain point. It is the measurement of the globe of the earth in 3D, and you can't put that on a flat map, hence projections. Projections are used to present a curved area of the earth on a flat map, and generally have an easting and northing rather than latitude and longitude. There are many different types of projections for different purposes, for example, one might preserve the size of land masses and another might preserve distances.

As for geographic coordinates, you will likely come across two in the UK. WGS84 is one of the most commonly used around the world. Consumer grade GPS will generally default to this, and it is the coordinate system used by Google Earth. The problem with WGS84 is continental drift. Different plates will move in different directions, so the same point on the ground will slowly change coordinates over time. The solution to this is to have a geographic coordinate system tied to each plate, so any given point on the ground wont change coordinates over time. the UK is on the Eurasian plate, which is covered by ETRS89. This system was created in 1989, and at the time was the same as WGS84, but the two diverge by 2.5cm a year, so the difference is currently about 60cm.

The most commonly used projection in the UK is OSGB36. This is the projection created and used by Ordnance Survey way back in 1936 for their mapping. Theoretically, a transformation from geographical coordinates to a projection like OSGB36 should be a simple mathematical calculation, but there is a problem. Back when they started surveying using OSGB36, the technology was not as advanced as today. Over long distances, slight errors crept in to the maps. When this was discovered, because of improving technology, OS had the opportunity to correct their maps, making the transformation from geographic coordinates to projection work again, but they didn't. What they actually did was to keep their maps the same, and create a database of local variations to the basic transformation (14.5Mb in size), which can be up to 20m horizontally. This set of corrections is called OSTN02 for horizontal corrections and OSGM02 for vertical corrections. Why is this a problem? There are variations in which pieces of hardware and software support it. Consumer GPS receivers almost certainly wont. Survey grade GNSS may do, but it is something worth checking, especially if you are buying old equipment second hand. As for software, Proj.4, the geographic transformation library used by pretty much all open source GIS software doesn't support it as standard, and neither does the standard commercial option, ArcView, though they do have an optional download which will allow you to use it. All of the digital maps for GIS supplied by Ordnance Survey are based on the OSTN02 corrections, so if you are surveying using a device that doesn't support it, you will be in the wrong place on the map. Most people in the UK will understand and expect OSGB36 coordinates, so a surveyor will have to use them, despite these problems. For the benefit of other surveyors, please also give ETRS89 coordinates in your geophysics reports. They will thank you for it.