What is ESRI's Terrain Service Elevation Layer World_0_M dataset? Notes on FMV tests and the Terrain service.

656
3
07-06-2022 01:26 PM
Labels (3)
by Anonymous User
Not applicable

This is a repost from here with added detail.

Using a drone equipped with a video camera the goal is to create Full Motion Video. I wrote some software to convert the my drone's telemetry into a Video Multiplexor compatible format. So far so good.

It's ideal to use ESRI's Terrain world elevation layer for quality and convenience, but there's a lot of uncertainty in the documentation and tool output.

For example, if you use the Terrain elevation layer as input for analysis with no definition query, there is no way to determine what underlying elevation dataset was used as tool input. The docs page tells us that the default behavior optimizes for "Best", where "Best" means smallest cell size (highest resolution).

This is inherently problematic if you care about data integrity. Were NAVD values used? EGM96? An uninformed user might use a tool like Extract Values to Points and have no idea that the vertical datum could be something unexpected depending on what "Best" means for their geographic area.

If you use the "default" Terrain service as DEM input to the Video Multiplexor tool, then you automatically encounter the problem detailed above. But you also encounter several compounding problems depending on flight altitude, relative camera angle, and camera internal characteristics (such as HFOV and VFOV). The Video Multiplexor requires a DEM that covers the entire flight area and calculated FOV footprint. This means that if you have a video taken at nadir, but even 1 frame shoots above the horizon, the tool still requires an almost continental DEM to succeed. Adding to this issue, the error message doesn't tell you the actual nature of the problem. It just tells you that it failed to extract elevation values from the DEM. Definition queries for semi-global DEMs such as STRM do not guarantee success here, because if the horizon appears in the video then the footprint is theoretically infinite. The docs do not explain how that extent is calculated.

I did lots of tests, such as forcing every frame's relative camera angle to each integer from 0 to -90. I could find no common denominator for success.

Finally on a whim, I tested the Terrain service's World_0_M dataset as definition query input to the Video Multiplexor tool. This input works in every case, but I can't find any documentation or metadata for this layer. The name implies a global fictional zero elevation raster. If you click to identify raster values in the Map it always returns 0 meters elevation. But astonishingly, the output FOV footprint shows elevation contours. What is going on here? I can guarantee Video Multiplexor success, but I have no idea what the underlying vertical datum or dataset is. Any links or clarity are much appreciated.

 

0 Kudos
3 Replies
RajinderNagi
Esri Contributor

I can't comment on FMV workflow but on Terrain service.

It's a multi-scale and multi-source service where different sources are combined to give a seamless output for visualization. However, when running analysis, you would need to filter a desired dataset from the service using "Make Image Server Layer" tool and set appropriate cell size, projection etc. If you add Terrain service from Living Atlas in Pro, it has pop-ups defined for definition query, which you can use. There is a export limit of 5000 rows x 5000 cols on the service and if your request exceeds, it would fail. World_0_M is dummy raster with elevation values of 0 used as background.
There is an old video covering analysis using ArcMap but applicable to Pro also.
https://mediaspace.esri.com/media/t/1_5gsooyj9

If you need to use elevation as ground surface in offline environment, you can Terrain 3D (for Export) https://www.arcgis.com/home/item.html?id=09ae40dd6766478bb7de3c557c1421cb to export tiles.

by Anonymous User
Not applicable

Thank you for this insight.

It's a strange situation with the FMV Video Multiplexor... using the dummy DEM produces the result that I would expect from the Terrain layer as a whole.

0 Kudos
RajinderNagi
Esri Contributor

Terrain layer has limit of 5000 x 5000 pixels like mentioned before. The tool might be fetching data from coarser raster but I am not familiar with FMV Video tool workflow to comment on. 

0 Kudos