Mapping Basics for Drone Ground Control
I have to assume you know nothing about the science of land surveying. Land surveying is a profession that requires a license. A licensed LS has the legal qualifications to map land ownership, land forms, locations of structures, etc. They have an obligation to do this work accurately and so their surveys will hold up in court. They do these legal surveys by measuring angles and distances from previously located control points. They also establish accurate benchmarks (vertical control) by using a sight level from known benchmarks.
In saltmarsh restoration, you will be mapping a land form with your drone and a GPS unit. This does not stand up in court and is not land surveying. You are creating a sketch of an existing condition of the marsh for documentation of conditions or work completed. A drone and a GPS can not equal the accuracy of or replace a legal surveyed map conducted by a Land Surveyor.
That said, for our purposes of documenting ecological monitoring and restoration, the drone and a good GPS offer accuracy that may be good for our purposes if a few tried and true methods are employed to provide the most accurate sketch possible. Here are a few tips.
A great place to start learning about the potential of drones to map your project is from the great people at Drone Pilot Ground School. They have an excellent web page that outlines how to get started and a comparison of some the software you will need. Click here it view it.
First, be aware that all maps have error.
It is your job to minimize that error. Land Surveyors have a degree of accuracy that they must meet to “stamp” their plan. And they have a tried and true way of measuring that error, by beginning and ending at the same point. You should do that too.
Land Surveyors use angles and distances to close their circle. You have a drone and a GPS. How do you measure your error? How can you assure DEP that your map is good enough?
You need to locate “control”. Whether it is for horizontal or vertical measurements, you need to establish control to measure how much error your drone and GPS are giving you. Your drone has a GPS on board to use for navigation. But how good is it? Are you accurate to plus or minus 10 feet or plus or minus 1/4 of an inch? And remember, a GPS’s vertical accuracy is 3 times that of its horizontal accuracy. So in this case, plus or minus 30 feet or 3/4 of an inch. It makes a difference.
The tool that you are going to use is your GPS to establish ground control points (GCP) in your mapping area. It has to be a good one and one that you know how to use. What is good? Well, for about $8000 you should acquire a GPS that has RTK (Real Time Kinematic) capabilities that will communicate with an established CORS network. (CORS = Continuously Operating Reference Stations) You are lucky because the Massachusetts Dept of Transportation operates such a CORS for you to use free of charge. You can use a unit that allows for post-processing of the differential correction, but that is a real pain when RTK is available and so cheap.
What to locate with your GPS?
Find objects that can be easily seen from the air: a post, a hydrant, a rock, a crack in the road, a bridge abutment, etc. I would stay away from power line poles. The energy in power lines can scramble your satellite signals and introduce error. Keep a record of what you have located, e.g. NE corner of large boulder, base of post, center of manhole cover… For points out in the marsh, use a target. I use the tops of white plastic 5 gallon pails but any clearly observable white object will do. You must begin and end your location of objects at the same place. This will give you an idea how much error the drone is picking up as the day progresses.
How many?
In order for you to see what kind of error you have in your drone map, you will need to georeference your drone map in your GIS project. Here is a link to a page that describes the various transformations that can be employed to georeference your orthomosaic or digital elevation model that your drone produces. For checking a map for its horizontal accuracy, at least 3 GCPs are required for a Polynomial 1 transformation. For a check on an elevation model, at least 10 points are required for a Thin Plate Spline transformation. Go to the link for a more thorough explanation.
