First Time User of GCPs

Flight - Discussion and Q&A
1

Hi all. How many Ground Control Points in necessary for processing a <20 acre area. Does the number change based on the area? Thank you.

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2

The first rule is to encompass the entire limits of the area. From there I would recommend that the points not get further than 600-700ft from each other. If you have an RTK or PPK solution on the drone then you really only need 1 or two in the middle but yes all of this depends on the size and shape of the site. Can you share a snapshot or KML?

3

Thank you for the response. I am going to map a rectangle and use 4 corners and 2 points through the long mid line. The RTX GPS I plan to use records elevations as height above ellipsoid. Does Drone Deploy support this vertical datum for GCPs?

4

If you are running RTK I would use the 4 corner and two checkpoints. The goal of using GCP’s with RTK is to create a plane to lock the site down and apply real world elevations. Without GCP’s DroneDeploy uses the ellipsoid height as calculate by you altitude from home. So when running standard GPS or RTK GPS without GCP’s you will have to use the single point elevation calibration in Map Details. If you are using GCP’s you shouldn’t have to do anything.

5

I have a follow up question. I am using a standard DJI Phantom 4. I processed a mission with 1100 photos with 80% front and sidelap flown at 100 ft AGL (no GCPs). The RMSE is 5.4 ft, X is 5.4 ft, Y is 6.1 ft, and Z is 4.5 ft. What kind of acuracy can i excpect if I utilize GCPs mapped with a sub foot Timble GPS? Is the map accuracy limited by the Trimble GPS or the Phantom GPS?

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6

When you say “sub foot” Trimble is it a handheld or a standard receiver (base/rover)?

Is there any reason in particular you are running 80% side lap? Most photogrammetrists recommend no higher than 70. Our default is 80/65. There is no increase in accuracy by running 80 side. It can actually cause problems by having too much data and all it does is make processing take allot longer.

The accuracy of the map is always held back by a non-RTK drone’s inaccuracy. GCP’s will get you down to sub-foot on checkpoints. Do you use checkpoints?