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.