DEM Exporting Validity

[tdepke]

Howdy again,

This question involves how that elevation value of a DEM is calculated if there are multiple polygons that exist for that particular projection. This could happen in real life if there was a cliff and an overhang. So, the DEM's that I am producing are not from aerial photos, but photos taken about 10m away from a stream bank where there is a nearly vertical outcrop of soil with a lot of boulders and cobbles at the bottom.

If there is a large rock in the foreground of the projection, will a DEM export the elevations of that rock, or the elvations of the bank behind the rock?

Here is some visual help. It is most helpful to look at the bottom right XYZ Axis to make sure you know what angle you are looking at, but the first picture is looking directly along the X axis so the Z-axis is completely Vertical, and the y-axis is completely to the right. In the second picture, the axes have been rotated so we can see behind this boulder, and that yellow "X" mark would not be visible if the DEM was produced looking along the X axis (like the first picture).


http://www.tylerdepke.com/wp/wp-content/uploads/2011/11/DEM-Q1.jpg


http://www.tylerdepke.com/wp/wp-content/uploads/2011/11/DEM-Q2.jpg

This would be very important for analyzing changes in volume of sediment and rocks from one time frame to another.

Thanks again,

Tyler

[Alexey Pasumansky]

Hello Tyler,

DEM export in PhotoScan takes into account only the maximum Z value for each pair of X,Y (if exporting in real world coordinates - Geographic tab in the Export DEM dialog). So the resulting DEM will contain only the rock but not the area under it that is not seen on the first picture.
In general selecting planar projection planes works in the same way.

[tdepke]

Thanks Alexey. That is really good to know for my particular application. I have a question regarding the angle at which one can export a DEM. I see that you can export it along all the different XYZ axes (with a bunch of different combinations). You can also export the current view and specify a marker. Would it be possible in the future to specify an angle between different axes? For instance, you could put in 45 degrees, and for this particular problem that I was having, that way the large boulder would not block the potential erosion behind it. That being said, there might be other parts of the image that would have two values, but I just think it could be something to think about for future versions.

Thanks again,

Tyler

[Alexey Pasumansky]

Hello Tyler,

I think we'll consider your suggestion. Thank you.