GCP Polygon Limits

GCP Polygon Limits - Part 1

GCP Placement is Critical!!!

This is the first part of a multi-part review of the importance of GCP placement and just how many are needed to get a good 3D solution.

2D orthomosaic requires minimal GCPs for a good solution. 3D requires more GCPs in strategic locations to get good results.

Objective: Is to determine just how many GCPs are needed for various camera/sensors/lens combined with the above ground level (AGL) flying heights for 3D mapping.

Testing site: A 5 acre rectangular parcel that contains brush, trees, buildings, dirt and paved roads.

Drone Platforms: Mavic 2 Pro, Inspire 2 using an X7/16mm lens and X5S 15mm/lens

AGL: 100', 200' & 300'

Overlap: Front 70%, Side 70%

For more details on the drone flight parameters see book "Flight Parameters" at www.cc4w.net.

Processing software: Metashape Pro by Agisoft

For more details on using Metashape Pro for 3D mapping, see books "Metashape" and "3D Road Intersection" at www.cc4w.net.

The first thing we are going to look at is the Neat Area (NA) for sensor/lens. The image NA is determined by the amount of side and front overlap of the flight lines. The NA is the image area with the least amount of distortion that will be used when stitching the images together in the processing software.

The amount of lens distortion will vary for each drone platform and will also vary between the same drone model due to the less stringent manufacturing process.

The higher end cameras/sensors such as PhaseOne camera will have less lens distortion however they are very expensive.

Good business practice requires balancing the operating funds with the Return on Investment (ROI). The biggest bang for the buck.

The testing and solutions found in this mini-series will review the most common drone platforms that we have access to as noted above.

Below is the image NA for the Mavic 2 Pro (M2P) at 200 AGL for the 5 acre site. The flight will require 70 images.

As shown below are the actual seam lines for the images after they have been stitched together by Metashape Pro, which mimics the image NA as shown above during the mission planning stage.

For more detailed information on the NA, review books "What's My Mission?" and "Time to Fly" at www.cc4w.net.

Five (5) GCPs were placed in specific locations on the 5 acre test site. One in each corner and one in the middle.

The outside corner GCPs were used for the polygon area of the model.

The images were optimized by using the four outside corner GCPs to constrain the model for the map projection.

The middle GCP was used as a check point to determine the amount of distortion of the model at this location as shown below:

As can be seen by the chart above, the amount of distortion was significant for all flight configurations except the last two.

The lens played a major role in the amount of distortion.

The middle GCP was then added to the map projection and optimized. The amount of distortion is shown below:

A good solid mathematical triangle is critical to constrain the 3D model.

Conclusion

It is recommended that at least 5 GCPs be set at a minimum. Their location is critical to providing a solid triangular mathematical solution. For larger parcels, more GCPs will be required.

For 3D Mapping, a general formula of "Acres x 0.2 (X7 or X5S) or 0.3 (Mavic 2 Pro)", depending on the sensor/lens/AGL can be used for missions above 17 acres. Five (5) GCPs at a minimum for all drone platforms below 18 acres.

The next part in this mini-series will look at what affect it will have by adding more GCPs to the model.

There are two training videos available on "3D Mapping" located at https://vimeo.com/ondemand/smms/

www.cc4w.net

"I fly Drones for a living mapping the earth one (1) acre at a time and write DIY surveying math/mapping books/videos for fun!"

Check out the New video series on "Survey Mapping Made Simple".

https://vimeo.com/ondemand/smms/

Sponsored by Cooper Aerial

Leaders in Modern Mapping Methodologies.

Cooper Aerial

A Free online "Flight Parameter" web app for multiple drone platforms is now available:

Click here to open.

If you like the information provided above and would like to see more in future email posts, then consider purchasing some of my books and videos to help support the effort. As all of you know, computer hardware, software and drones costs money. I put 100% of my book/video sales back into the business to write more books, produce videos, buy drones to test and so forth.

Thank you all that have supported this effort. It is greatly appreciated.

www.cc4w.net

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