Hello Hugh,
You may want to consider the following tools for your type of project.
1) use a small 30metre steel tape in a rolling winder (approx. cost $30) to create scale lines for your project. Depending on the scope of your project, you could typically layout three control markers on a straight line with your tape. Then create two other control markers by using the mid marker and each end marker by setting two markers on one side of the 3marker line. Tape the distance from the end marker and the mid marker to the two point outward from the 3point line. These are the distance observations for scale.
2) Use a second hand builders level (very inexpensive) and a surveyors rod to measure relative elevations of all five markers. These are the elevation difference observations.
3) use a hand held GPS (everyone has one right) to record averaged positions (N,E) for the five control markers. These are the coordinate observations.
4) If you set out the three points on line properly carefully, then you have an angular observation being 180 degrees at the midpoint
5) Enter your observation data in the excellent free Gama Least Square adjustment program by Ales Cepek and team. Use the appropriate standard deviations for the observations. For example for the 180 angle observation use "0" seconds thereby fixing the straight line. Maybe use one centimeter for the distance and the elevation difference observations. Use two to ten metres for the standard deviation of the autonomous GPS. You will first run a separate vertical adjustment with the elevation difference observations and thereby obtain adjusted elevations and have a check on the quality of the observations. Secondly you will run the horizontal adjustment. The Gama least square adjustment will respect the weighting that you used and provide final real world coordinates and relative accuracies. You could even add in forward and reverse compass azimuths with say a standard deviation of about 2 degrees, but not needed. The adjustment will show you how good your autonomous GPS observations as are as well. I think typically around the 2 metre absolute positioning level and Gama will make the distance observation best fit the GPS positions of course.
6) When you add the control points in and run Agisoft, the result will of course show how well the control fit the photo alignment/mesh which provides another indication of the accuracy of the input scale measurements.
This may seem overly complex to do, but once you have the abstract format understood, it is very quick to do for these small networks and very rigorous way t do it.
Gnu Gama can be found here:
http://www.gnu.org/software/gama/I am not sure whether this user interface for Gama up to date and working
http://roci.sourceforge.net/I created a little resource site about ten or fifteen years ago that has some sample input file and resource info on using Gama
http://vrmapper.com/gama/gama.htmlThere should be sample horizontal and vertical input files that you can simply use and fill in your own data. There is an instruction on how to install and run Gama as well. It is a very good/powerful and easy program to use once you complete your first project. I think it is ideal for use on small archaeological or similar sites.
I hope this helps,
Jim