AMSL contour lines from DSM/DTM process

whats the process to getting above sea level contour lines using a dsm or dtm? If I use a accurate GCP files will it use the geo_z value to represent altitude? I am fairly proficient in QGIS, but am unsure of how to get ASL data into the DSM files. thanks for any input.

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What elevation are your image data in?

the altitude in the exif data of the images is in meters.

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Do you know if it is above MSL, above takeoff, or another vertical datum?

drone imagery, so AGL relative to take off location.

You mean DJI imagery? Not all platforms use above takeoff. My stuff is above MSL, for instance.

Hmm… You could use an EXIF editing tool to convert the heights from above takeoff to MSL using your home point altitude from your flight log.

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Yes, DJI imagery. Ill give it a shot. Appreciate you Sir.

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My M2P EXIF data is AMSL.

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And so it continues to be as clear as mud, haha. All the M2P stuff I’ve gotten is above takeoff. Ugh… Different firmware revisions? Different flight planners? I don’t know what determines it at this point.

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I used a digital photo exif scanner which differentiates the absolute altitude and relative altitude on images from different data sets and am getting conflicting values.
On one dataset the images have nearly the same absolute and relative altitude and on another dataset it looks like absolute altitude is close to being correct to the location and relative was correct for flight height. So yeah clear as mud, will continue to try and figure this out.

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Thanks for digging.

Can you remember if you changed firwmare versions or changed flight planners for the different datasets?

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I dont belieIve I upgraded firmware and I use the litchi app for flight planning. When I get home I am going to run a short flight , will take a few images before takeoff and compare the elevation data to the in flight image elevation data to hopefully clear things up a bit. I am flying a p4p btw. I just do this as a hobby, not professionally as you can probably tell, but I do like learning new things concerning tech.

Thanks for the help.

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Hmm… Maybe DJI switched from AGL to MSL or vice-versa at some point, or uses one or the other depending upon product “tier” (consumer vs prosumer/pro).

That sounds like a great test! Thanks for taking the time to do it.

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Maybe they all took off from the beach where AGL = AMSL! :slight_smile:

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I found that the images exif altitude data is indeed ASL. However the values I am getting are wildly inaccurate if you consider google earth to be accurate. There is as much as a 40 meter difference between the two. my image data was at 217 meters altitude while google earth says about 180 or so for my location.

know of any apps to edit exif data recursively?

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I will be a broken record here but:
Google Earth is not truth data.

Do you have a local municipality GIS website that offers terrain data as DEM? We have them available here, for instance, and yeah, way different than Google Earth in many locations.

Another acceptable proxy might be the latest NASA SRTM Data in 30m.

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The radar altitudes taken from orbit appear to have a variable offset from the visible light imagery taken from planes (all the high res imagery on Google Earth). At the top of my quite steep hill it is 30-40m, which also introduces up to 20m altitude error.
I don’t imagine that is the worst case.

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Which shows the dataset I just processed as more than 20m below sea level because it is an area of the world not well served by WGS84. :slight_smile:

It’s all messier than anyone would expect from the outside, isn’t it?

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It is the worst, haha. When are we getting the 10m rework with all the cleanup they are supposed to have done!?

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:man_shrugging:

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