GCP Polygon Limits - Part 2
 Part 2 for GCP Placement!
 Here is the link to Part 1: https://conta.cc/2Ut13eH
 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.
 Five (5) GCPs were placed in specific locations on the 5 acre test site. One in each corner and one in the middle.
 A good solid mathematical triangle is critical to constrain the 3D model. See Part 1 for a discussion on the proper placement of the GCPs: Here is the link to Part 1: https://conta.cc/2Ut13eH
 As discussed in Part 1, understanding the image Neat Area (NA) is important to get good solid overlap to minimize the lens distortion for each image.

﻿For more detailed information on the NA, review books "What's My Mission?" and "Time to Fly" at www.cc4w.net.
 Check points are needed in order to calculate the RMSE values to determine the XYZ accuracy and how they rank in the ASPRS positional standards.
 The image above shows mulitple check shots that are spread throughout the 5 acre site. Four (4) check points would be sufficient to meet the ASPRS mapping standards. Extra check points were set randomly to get a thorough review of the aerial triangulation solution.
 The RMSE values of the 27 check shots were averaged and entered into the chart above.
 All three drone platforms did really well for the X & Y accuracies. The distinction between the three drone platforms for the Z value is where it gets interesting. The M2P did well at 100 AGL. The X7 & X5S sensor did well at the 200 AGL flying height.
 Conclusion The X7 sensor is the clear winner at 200 AGL for the Z value together with the proper placement of the GCPs. Depending upon the accuracy of the deliverable for 3D mapping, will determine which drone platform and flying height will be needed to meet the requirements of the scope of work. Part 3 of this series will take a look at the amount of distortion that occurs outside of the GCP polygon. 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".