Question on DEM Accuracy in TG2 When Compared to Actual Features

cbshort · January 31, 2013, 02:41:18 PM

I have a DEM terrain (approx 100 hgt files combined) with which I converted to a .ter file via 3DEM. In 3DEM I selected a small portion of the entire group (an island off the main country) as the final TER file. I loaded the TER file into TG2 and was doing visual comparisons with actual images of the same location. Attached are two photographic views from the same location with approximate render equivalents. The views in TG2 resemble the locations ok but the actual features are more "molten" or eroded like along with some fractal like details in the default state. You can see many of the real mountains in the photos have sharper peaks and such.

The question is this, is the resolution of the DEM causing this or do I need to do something else other than just loading the TER file. (I did attach a fractal with very small features that does not alter the main features. It look basically the same without it). Also, I am noticing that the elevations may not be right when I right click and check the altitude of the selected. Not sure if that needs to be adjusted in 3DEM prior or in TG2. Thanks for any insight!

Chris

NOTE: The Comparison files are the photos, the testing are the raw TG2 renders.

cbshort · January 31, 2013, 03:02:56 PM

Re-did the first view with similar lighting to the comparison photo.

jo · January 31, 2013, 04:28:01 PM

Hi,

Is this SRTM data? That isn't very high resolution. Most of the data for the world is about 90m I believe and the US and some other parts of the world is 30m. I would say that's the reason things aren't matching more closely. I have some 8m data for part of NZ and although I don't know the location well it's obvious that there is a lot of detail and fidelity in the DEM.

Check this out:

http://www.planetside.co.uk/forums/index.php?topic=15359.0

This is done with what is basically a very high-res DEM.

Regards,

Jo

cbshort · January 31, 2013, 04:40:10 PM

Yes it is SRTM data from the Viewpoint site. I don't believe there are any higher versions of this location available. (I have some GEOtiffs but they look similar).

Is there any way to improve this with terrain operators and such? I did find the Fractal controls that removed the random fractals on the terrain (thus making it smooth) but would like to know if there is anything else that can be done to improve the features.

I'll look at your examples as I'm sure DEM resolution plays a part..

Tangled-Universe · February 01, 2013, 05:24:48 AM

Jo is right about the lack of resolution in your DEM data, I'm afraid.

You can spice up your terrain by adding extra powerfractals.
I'd start with large scale displacements (100-500m feature scale with 20-100m displacement factor).
Then add another fractal with smaller scales and by doing this adding in smaller details with every fractal.
Say 3-4 nodes max.

Another option is to import the DEM into World-Machine (WM).
There are some options to make the terrain "swell" in WM.
This is useful because you can then erode the 'swollen' terrain back to its original size, but then with very nice erosive details and such.

Actually that would be my approach, do WM first and then adding powerfractals in TG.

Quote from: jo on January 31, 2013, 04:28:01 PM
I have some 8m data for part of NZ and although I don't know the location well it's obvious that there is a lot of detail and fidelity in the DEM.

Can you share that data with us Jo?

From which region is it?

Cheers,
Martin

cbshort · February 01, 2013, 07:38:59 AM

As I suspected. At least is is adequate for aerial shots. One question about 3DEM, the image I see before it is saved as a ter file looks much better. Are there limitations with the ter file it saves? Better ways to save it? What would be the optimal format to bring into TG2?

Thanks!

cbshort · February 01, 2013, 04:13:15 PM

I was able (with more digging) find a higher resolution GEOtiff of my area in question (I'll post examples over the weekend). It does show improvements to the previous DEMS I was using but now I have GEOtiff questions.

The previous DEMS I was able to select them all at once and load them into 3DEM and export to ter. With the GEOtiffs, I can only load 1 at a time and don't see a way to combine the 3-4 GEOtiffs to make one clean mapp of my intended area. My goad is to avoid having to stitch them within TG2. Am I loading the GEOtiff file incorrectly? To be specific, they are the Aster Global Dem V2. Any suggestion as the best way to use them (whether with 3DEM or directly into TG2 or other methods) would be welcome.

Oshyan · February 01, 2013, 07:11:29 PM

3DEM is pretty old and no longer updated. I wouldn't be surprised if it just can't handle the job with GeoTIFFs. Loading them natively in TG should work, if they're georeferenced they should align properly. I'm not sure if WorldMachine supports GeoTIFF natively, but like Martin my first suggestion for improving the terrain would be to "swell" then erode, so you might kill to birds with 1 stone of WM can do GeoTIFF. Other than that, other dedicated GIS-type software, both free and commercial, is your best bet, something like Landserf, GRASS GIS (both free), or GlobalMapper (not cheap, but pretty much the best product of its kind for working with real-world data).

- Oshyan

cbshort · February 01, 2013, 09:53:59 PM

I do have Grass GIS but havn't a clue how to use it. Will have to read up. I will look at world machine but my initial play with my new DEMs and 3DEM settings have improved.

cbshort · February 01, 2013, 11:06:27 PM

I did try to load the Geotiff in directly to TG2 but it told me "convert image to SGI for non standard bit depths"? What does this mean?

Oshyan · February 01, 2013, 11:08:13 PM

Is your GeoTIFF 16 bit or 32 bit? I believe they ought to be 32 bit, or at least that's what TG expects. A 16bit TIFF will give that error message.

- Oshyan

cbshort · February 02, 2013, 11:21:04 AM

I'll try that conversion.
I also am trying the two week trial of Global Mapper. What format would be the best to output my maps to bring into TG2?
Thanks!

Oshyan · February 02, 2013, 03:41:14 PM

32 bit GeoTIFF or Terragen .TER (the latter does not have georeferencing).

- Oshyan

jo · February 03, 2013, 07:38:21 PM

Hi Martin,

Quote from: Tangled-Universe on February 01, 2013, 05:24:48 AM
Can you share that data with us Jo? From which region is it?

The NZ data is not actually the data I thought it was, it must have been some other DEM. I was looking at a lot a while back! In any case, the NZ data comes from here:

http://geographx.co.nz/what-we-do/map-data/downloads/

The 8m data is interpolated from 20m contours so I guess it has 20m vertical resolution but potentially higher horizontal resolution. The first DEM is of Lake Rotorua. I'm not sure about the second.

Regards,

Jo