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Advise appreciated on this first project

This is not my very first mapping job, but close. This is the first time I’ve been on my own registering my own GCP’s (using Emlid), and the first time I’ll be using Pix4D mapper for a paying project. Of course the project looks pretty messy to me since I’m a rookie at this. So… I’ll appreciate all suggestions on this project, from planning and capture to procesing.

An environmental engineering firm wants mapping files to assess the volume of a decades old coal ash pile. The pile is about an acre in area, about 50-ft high, has a largely flat top, almost vertical sides, and trees/brush growing up in the sides. I’m attaching a couple of photos I took today during a prelim assessment. The pile is stable to foot and vehicle access.

I’m planning to use 12 GCP’s with about half on the pile and half on the surrounding grade. Will keep one or two to judge error. Based on MOCA, I’ll be flying about 120 feet above the top of the pile. Thinking I’m looking at a double grid plus some obliques and orbit. Shooting with a P4P. I believe that I have decent view of the surface on 3 sides and will have to make a SWAG estimate of the west side edge in the tress. Will see if I can upload a couple files showing two sides. Oops - new user so can only upload one. All comments & suggestions invited/welcome. I expect to be doing this project next week. Thanks All. Regards… R.


My experience with a P4Pro is that obliques actually added more error vertical error to my mapping projects than without them. In addition to your GCPs you also need to collect checkpoints in order to assess the accuracy of the DSM or DTM. You can use a few of your GCPs to asses horizontal accuracy. I typically collect an additional 20-30 topo check points to assess vertical accuracy. Also, I would process with and without the obliques and see which is more accurate. You may get different results than me, especially with the nearly vertical sides.

For mapping a double grid is usually overkill. A single grid with 75/75 overlap will be just fine. At 200ft agl, I can easily get vertical accuracies <0.1 ft with this method. The key to high accuracy is very good quality images. You must set manual focus to infinity and a shutter speed of at least 800 to have sharp photos. I manually set these camera settings with iso set to auto. Sharp, properly exposed images result in more accurate data. You can have great survey data but if your images are blurry your output won’t be very accurate.

You also need to ensure that you follow good surveying practices when collecting GCPs and checkpoints. If you do all aspects of the project correctly, the P4Pro can produce very accurate results.

Good luck!