DEM resolution, v2.5

[Larry]

I'll bet the answer to this question is somewhere in the documentation, but I'm lazy.  

I'm guessing that the auto DEM download gets the 100 meter resolution DEMs. My existing DEM files are also 100 meter. Is it possible to use the 30 meter DEMs? Can they co-exist with the 100 meter, say for an area of special interest?

Would it be possible to be able to select 100 meter or 30 meter for the auto download feature?

And if one can use the 30 meter stuff, would it actually help the display on the screen?

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Larry

[Krein]

Yes, there is 30 and 90 meter data available from auto download.

The world dataset is 90 meter SRTM, while the US only stuff is 30 meter NED dataset.  There is a huge difference in the quality between the two.

They can coexist in the autodownload, but in the DEM file structure you'll either get one or the other if you have DEM files that overlap each other.  

There's also 10 meter data out there.  If you use the "use DEM files if available" feature, the 3d code will first use any DEMs and default to downloading 30 meter US data if that fails.

[Larry]

Thanks Krein,

A 'typo' in my questions:   '>

Replace meters with feet.

I was hoping that I could use 30 foot resolution DEM data on small areas I am interested in, even though that data would overlap my large 90 foot resolution file.

Seems like a lot of work, but maybe I could temporarily rename the 90 foot res DEM file to force the use of the  30 foot res.

Is there an easier way?

Larry

[Krein]

I personally haven't noticed much of a difference with the 10 meter data, though I haven't done a thorough comparison.

I'll check on what it does with multiple files, though.  It might take the highest resolution one anyway, but the other author wrote the code, so I'm not sure.

[KanyonKris]

I've been playing with the 3D features of 2.50 and I like the auto-download feature, but it would be nice if TF could use any available elevation data to render 3D topography. Let me explain:

Currently in the "3d Options" dialog there are three settings for "Data source":

- Use DEM files if available
- Always download (US only)
- Always download (World)

If I have DEM files selected I see 3D topography in the DEM coverage area but flat (sea level?) topography outside this area.

But if I switch to download US I will see 3D topography everywhere (unless the data doesn't exists on the server or the server is unreachable).

Why can't I set TF to use DEM files and then for areas outside my DEM coverage have TF auto-download the missing elevation data? Is it an issue with creating 3D meshes (polygons) with two different densities? It shouldn't be that hard to do.

As the icing on the cake, it would be nice if TF reported what elevation dataset is in use in the 3D view (under the blue center ball). Maybe down in the status bar.

Krien commented that he doesn't notice much difference between 30m and 10m resolution - but I sure do! I live in Utah and mostly use TF to "virtually explore" areas in southern Utah which has some rather extreme topography in places. The 10m data dramatically improves the visualization of these areas.

Here's a view of Angel's Landing in Zion National Park using 30m resolution elevation data:



Now here's the same view with 10m data:



Pretty noticeable improvement to me.

[Krein]

Quote (KanyonKris @ Mar. 07 2005,10:17)

Hi,

The intention of the "Use DEM files if available" option is to have TopoFusion use DEM where available, but switch to auto-download when DEMs are absent.  Is this not the behavior you are getting?

If you construct a 3d model that is partially covered by a DEM, you will get "flat" (minimum elevation of the dem) in the areas not covered.  This may be what you are seeing.  It does the test using "getElevation" at the upper left corner.  If there is a DEM for that corner it will use DEM elevations, otherwise it downloads the data for the whole scene.

Given the limited applicability of merging both for a single scene, it is unlikely we will ever implement it.  It is harder than it seems.

Even though I have gigs of DEMs (all of AZ) I usually leave mine on auto-download US, since I often like to do large views that take long to load in dozens of DEM files.

Thanks for the comparison of 10 vs 30 meter.  I can see a difference, but it's not enough to make me want to take the time to download 10 meters except in extreme areas of interest.

Unfortunately there is no code that selects the 10 meter over 30 meter data, so it's arbitrary as to what you get.  I'd just remove all your 30 meter, then you'll get 10 meter in DEM mode, otherwise the NED 30 meter stuff from the NASA server.

[KanyonKris]

Krein, thank you for reply and the info on 3D - good stuff.

Yes, I was seeing the "flat" areas because the 3D view was only partially covered by an available DEM so the rest was flat.

I'll take your word that rendering a 3D scene with 10m DEM data mixed with 30m auto-download data is not easy and may not be worth development time. Still, perhaps you can throw the idea onto the wishlist.

Just some feedback: it seems cumbersome to be locked into a DEM, US or World mode especially since there is no fast way to switch between them. Perhaps hotkeys could be defined so you can switch between DEM, US and World and then press "R" to re-render the 3D scene. Better yet, what if TF used the datasets more intelligently based on zoom level. You are correct that 10m data is overkill for viewing large areas and it takes longer to render because so much data must be loaded and processed. So how about switching from 10m to 30m elevation data once the user zooms out past some point - much like you do for the 2D datasets (1m, 2m, 4m, 16m, etc.). Seems natural to me that 3D operates similar to 2D mode in this regard. Maybe even have a slider like for 2D so the user can set this switch-over level/threshold.

Lastly, I agree that with the new auto-download of 30m data, there isn't much sense in gathering 30m DEM files. I only have 10m DEMs in my collection.