Resolution of download/cashe DEM

[Larry]

Hey Scott,

Just got to wondering if the download/cashe DEM resolution was as good as the stored ones on my HDD. These were generated some time ago, using the instructions on the website. Turns out that the original stored DEM files have a much higher resolution as compared to the download/cashe version. At least on my few tests. The later produces a very blocky 3D model when zoomed way in.

I was under the impression that the download/cashe files could create the same DEM resolution as before. What's going on do you think?

Larry

[ScottMorris]

Hey Larry,

The downloaded DEM data is 30 meter resolution -- the NED (National Elevation Dataset) data from USGS.  You might have 10 meter data, but most DEM is 30 meter.

If you're seeing blockiness, make sure you aren't on the "Force SRTM" option in 3D Options.  That uses the 90 meter worldwide dataset.  Otherwise, maybe post a screen grab?

Thanks.

[Larry]

Scott, checked to make sure I'm using the download/cashe option. As I remember, my NED data was the 30 meter stuff.

Here are two screen clips, the first using the download/cashe data, and the second using my DEM files:

http://www.larryscascaderesource.com/pics/downloadcashe.jpg

and

http://www.larryscascaderesource.com/pics/oldstoreddem.jpg

To my eye, the second is much smoother.

Larry

[ScottMorris]

Larry, can you fix the permissions on those files?  I get a "forbidden" message.

Thanks.

[Larry]

Scott, guess I should have checked to make sure the links work; I got the same error as you. I don't know what's causing it.... I think I remember some setting somewhere about opening a file linked from another website....

However, I found I could 'save target as' and save the picture to my desktop. I'm confused! 

But perhaps for now you can just copy the URL and paste it into the browser. That worked for me.

Larry

[ScottMorris]

Got it -- I see what you are talking about.  Not egregious but definitely an error.  It may be something TF is doing -- the interpolation scheme.  Can you double check what your interpolation scheme is?

The other possibility is just loss of info in the server's conversion to PNG.  I don't think it is loseless.  But I'd like to check the interpolation thing.

[Larry]

Scott, I played around with this a bit more. In both cases (stored DEM and download cashe) the interpolation method was set to Bicubic. I can't tell for sure, but it appears that the stored DEM is not higher resolution. I don't see any additional terrain on steep gullied hillsides for example, but it seems as if the 3D simulation using the download/cashe is made up of flat tiles (creating facets) where as the stored DEM 3D simulation seems to bend or form each tile such that there are no visible facets. I hope this makes sense.

But I also discovered that my stored DEM seems to be off by perhaps 150 feet in the central Cascades. This by looking at steep ridgelines with knife edge crests and noting the displacement of the shadow (the real shadow) on one side. The download / cashe is right on.

Larry

[ScottMorris]

Interesting that your data is off by a bit.

I think what is happening is that the interpolation setting doesn't apply to the downloaded DEM.  It's always bilinear interpolation, which gives the patch effect.  If you set to bilinear you should see similar results.

I can't remember why I didn't add the other methods, but there must have been some issue with it..  I'll make a note to check it out.  Thanks.