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Ground Control Points - Where & How Many

Hello -- Is there a rule of thumb for determining if Ground Control Points (GCPs) are needed or suggested?  I'm thinking it may not be needed for flights covering smaller areas.  Does the degree of elevation relief factor in to whether GCPs are needed or not?

If GCPs are needed, where within the survey area should they be placed and how do I determine how many to GCPs to GPS?

I'm flying a Phantom 4 Pro.  The GPS unit is a mapping grade Trimble Geo 7x and after post-processing, the horizontal accuracy is sub-foot.

Many Thanks!

Osprey

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For the Manual Rereferencing process to work you need to have the GCPs spread throughout the area, away from the edges, be taken with a high accuracy coordinate source and have at least 5 but not too many (20 or so). 

If you just want things to line up nicely, you can do what we do most of the time. We don't use a GPS at all to create our GCPs we test with. You can right click on the maps in your Map Detail page to get the coordinates on the screen. If you hide the layer you created and look at the basemap you can choose features to use as GCPs and get elevation values from Google Earth. This makes your created maps line up perfectly with the basemap.

Zane
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