Elevation differences between maps

Flew two missions on the same farm, same day, same flight altitude. Fields were on opposite sides of the farm lane. Property there is very flat, very little elevation differences.

Went into map 1 and selected a location next to the lane. Got 64.61 feet elevation
Went into map 2 and selected a location on the opposite side of the lane as the point in map 1. Got 45.23 feet. A difference of over 20 ft, which knowing the topology this isn’t correct.

I combined the photos from both flights into a single map and selected the same locations. Got 60.65 for location 1 and 59.95 for location 2. This is what was expected.

Can anyone comment on why the elevation would be so different from two different maps? I have the need to compare elevations between points in different maps and would expect the elevation calculation to be accurate.

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You need to use GCP’s or adjust the height of your maps. The elevation recorded in the Exif data of the photos is, in the case of a P4P or similar drone, taken from the on-board barometer. This typically provides 10X better relative accuracy during a mission than GPS (without RTK correction). But if the barometric pressure changed between flights, then the elevations will shift. To partially fix this use the elevation adjustment tool in DD. I say partially as it will not correct elevation changes introduced by barometric changes during the flight.

Or it could be something else related to the photogrammetry process. But for all of my maps of the same area flown on different days this has definitely been the problem. A simple shift in the elevation of the maps so they agree at 1 point nicely aligns the elevation over the entire map.


Good suggestions. Thanks.

The flights were the same day back to back so there wasn’t a great change in bar pressure. Nice sunny cloudless day. I do intend to do a GCPs for reference going forward.

0.001" Hg = 1 ft approx. so only a 0.02" Hg change would account for the 20’ difference.

I just learned something very valuable. A sound reason to use GCPs to confirm altitude points.

What I do find interesting is that when you process the flights separately you can get one elevation reading. Meaning I flew two adjacent fields as independent flights and processed the images as separate maps. When using the annotation tool to determine elevation I get the major difference between locations. But when I process the joint set of images as a single map there is no difference. That would mean that the barometer readings are not that far apart.

Either way, your suggestion of setting a control point is the best way to handle this.

Do the 2 sides of the lane appear in both sets of photos?

No. There is overlap on the edge of the flight though that includes the lane. Took a look at a point in the overlap in both flights and there was a 20+ ft difference.

map 1 - image
measures 45 ft

map 2 - image
measures 63 ft

Similar differences if I take points not on both maps.
Combined maps - image
all points show similar at 60, 62, 61 ft

Is there a way to adjust/calibrate the elevation in the DD app?

You can move the entire map up or down with a utility in DD. Click on the Map Details button in the side panel and then select Calibrate under the Map Tools section. Drag the calibration icon to a point with known elevation. Then enter the known elevation in the custom elevation bar and click Calibrate. You will see a confirmation message at the bottom of the screen.

Here is a link to this information: