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Ground control points

What handheld gps unit would you recommend to get ground control points?

Adam

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The Trimble GeoExplorer 6000 is a super pro tool, but it is really nice and gives great data and logging capabilities. It also costs a couple of thousand dollars... There is no end to how much money you can spend on this stuff.

Anything in the Garmin GPSMAP series would be a good option for most purposes and they are much more reasonable.

The maps are very accurate on a relative basis except for around the edges or in the presence of off-nadir images. The georeferencing process is there to take that relative accuracy and apply some absolute world accuracy to it.

The main reason for taking manual GCPs is that there isn't a sufficient basemap available for that area. Most of the US and Europe has good enough basemaps to be able to get a point within a foot or two. Most of the time that is enough. In the case where basemap coverage is bad (greater than 1 meter per pixel) it gets hard to get decent point placements using the basemap workflow. That is when you want to get manual GCPs.

Good question.

If you have access to that hardware, have at it. The accuracy of the map is only as accurate as the spatial data will let it be. A lot of users spray paint numbered X's on the ground and take a reading at the center of the X. They then use our manual georeferencing tool to associated the GPS coordinates with pixel locations in the images they collect.

Tudor
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