Relative Accuracy without GCPs?


#1

Recently I did a construction site. Shot it a 1in/pixel GSD with a Phantom 4 Pro. Frontlap 75%, Sidelap 75%. I gave the orthophoto (GeoTIFF) to the Land Suveying company that surveyed the property and the photo wouldn’t line-up right when they brought it into Carlson Survey. It was fairly close but a lot of the building edges in the photo were quite a bit off from the survey drawing, a few feet off.

My understanding that is that relative accuracy should be 2-3 times the GSD. So if I shot at 1in/pixel, then things should line-up within 3 inches, no?

We didn’t use ground control points because this was actually an inspection job for the lender to view the status of the project and the Surveying Company that was working with the lender neglected to tell us upfront they wanted to overlay it on an existing survey map. However I’m trying to figure out what the cause of the inaccuracy would be as I would think 75/75 overlap and 1in/pixel GSD should produce pretty good relative measurement accuracy. Yes, the geolocation coordinates in the GeoTiff would be off since we had not ground control points, but I would think you should be able to manually align it.

We were joking that maybe the builder didn’t build the buildings correctly! But we are assuming they did. The site was only about 5 acres.


#2

Hi @consultant,

I suggest taking a look at these resources for detailed explanations of accuracy in mapping:

  1. How Accurate is my Map?
  2. Accuracy in Drone Mapping: What You Need to Know
  3. Drone Accuracy & Cloud-Based Photogrammetry

Hope it helps! :slight_smile:

Cheers,
Christina


#3

@consultant,

Do you have more details on the mission? Altitude above takeoff point, overcast or sunny day, flight speed, shutter speed?

Regards,
Terry.


#4

It also helps to get some overlapping oblique images on flights to add to accuracy. A POI circle would help with 5 acres.


#5

In my experience a “few feet off” when mapping without GCP’s is quite normal. You are never going to get relative accuracy down to a few inches without GCP’s. I had a look a few recent flights and orthomosaics I have processed (without GCP’s) and they are out anywhere between 4 and 20ft. If you need accuracy GCP’s are the way to go and of course have the advantage that subsequent flights can be compared. I flew and processed a 100 acre dock facility last week with GCP’s and the x/y error was about 0.5 inch and z even less.

Another point - RTK equipped UAV’s. I am based in the UK and the UAV operators/surveyors I know still use GCP’s when they have RTK equipped UAV’s as without them they are not getting the accuracy.

Jeremy


#6

This is contradictory to the information posted by the leading photogrammetry cloud processing services. But this is actually what we experienced with a 1.5inch/pixel GSD, 75/75 frontlap/sidelap using a Phantom 4 Pro and the Land Surveyor said depending on where on the photo you examined it was off by 2-10 feet when trying to line it up with their survey drawing. We are currently running some experiments to ascertain if this was a one time glitch or we have an issue with equipment or in the processing workflow.

If our experience was typical that it makes no sense all the info published out there says stuff like this:

“Relative accuracy: Is typically a multiple of your data’s average Ground Sampling Distance (GSD). The horizontal relative accuracy is 2x GSD (for example, if your GSD is 2 cm/pixel, the horizontal accuracy will be approximately 4 cm) and the vertical relative accuracy is typically 3x GSD.”

“Generally, one can expect an error of 1-3 times the Ground Sampling Distance (GSD) of the original images for the relative position of a point in a project that is correctly scaled and reconstructed.”

“In general, the final accuracy for a model in X and Y is 1–2x the GSD, and 1–3x in the vertical however you will find that the average error tends to be closer to 1x GSD for all axes.”


#7

Make sure there isn’t a scale factor or a local coordinate system.