Digital elevation models - why are there have negative heights?

[ChrisC]

Hello, I am a little confused why the DEMs I generate have negative values? See attached example where the height ranges from 1 meter to -4.58 meters. Is it because photoscan picks an arbitray point and then every thing is either above or below that point?  Would it be wrong to add 4.58 to all values?   Thanks for any help on this!

[Dave Martin]

Chris,

How have you input height values into your project - do you have Ground Control Points (GCPs)? or are you relying on camera positions?

If it is just camera positions, I'm going to hazard a guess that your UAS might be a DJI? - there is a well-known issue with DJI whereby they load altitude above take-off point - have a search on the forum for    DJI height   for more info and means of fixing this deficiency.

Dave

[ChrisC]

Chris,

How have you input height values into your project - do you have Ground Control Points (GCPs)? or are you relying on camera positions?

Hi Dave, sorry I should have gave more info. This is an underwater area of coral reef. We had a diver taking the photos underwater, so we have no coordinates for true GCPs. But we do have several 30cm by 30cm cards the divers placed throughout the plot, so I am able to create pretty accurate scale bars, which I believe is what Agisoft is using to create the DEM.

Thanks for your help! 

[Alexey Pasumansky]

Hello Chris,

If you are using only scale bars and no absolute coordinates are given for the reference points, then you would get only relative dimensions in your project.

So it could be -1 to -5 meters, or -101 to -105 meters or 10 to 4 meters.

[ChrisC]

Thank you Alexey! 

[Alexey Pasumansky]

Note that you may need to adjust the model orientation in space using Rotate Object tool to be sure that the projection plane fits your expectation (for example, make XY plane of the model parallel to the real sea floor plane). After such adjustment is performed DEM should be re-built.

[ChrisC]

Thanks for the tip, I will definitely check on the orientation!