12 replies to this topic

[rok]

Hi,

I am using a DEM in tiff format (free 30m GDEM ASTER data) to create a terrain base map (for an area appx. 80x50 km) on which I will later add other data.

The plan was to:
1) create a hillshade from the DEM in ArcMap and export image
2) apply desired classes and colour ramp to original DEM and export image
3) overlay both in Photoshop

I see there are numerous topics on how to adjust the hillshading/relief shading for specific needs.. It's all very interesting and I'm looking forward to going through the basics someday (reliefshading.com, shadedrelief.com etc.) and then tweaking the process. BUT at the moment I'm interested in getting my base map, so I can focus on the data that comes on top of it.

I'm quite happy with the initial result, except for the grainy look of the hillshade image. For example, the depression at the top of the image should be flat, but instead looks like shattered glass - giving impression of anything but a flat surface.. I've tried playing around with the settings but had no big success.

Would be happy if anyone could point me to the right direction: should I look into data manipulation before hillshading or leave the data and work on the hillshading process itself? or maybe the data used here is not appropriate for my scale and I should be looking into the photoshop effects?

Thx in advance!

Note: I'm att. images of the original DEM, the DEM after hillshading and the final result from photoshop (not the whole area):
 dem.png   290.11K   40 downloads
 hillshade.png   449.74K   66 downloads

[Igor Brejc]

Hi rok,

You could try smoothing the DEM using Sun's denoising algorithm (http://personalpages...chell/mdenoise/)

[rok]

Hvala Igor

The Sun's denoising algorithm looks promising... but I can't make it work.. Using GDAL (via FWTools) I prepare the DEM file as described in the link you've provided (reproject data to projected coordinate system and then convert it to ESRI ASCII grid format).. but when I execute the command to use the algorithm the process crashes (This application has requested the Runtime to terminate it in an unusual way.) I have repeated the whole process with the example data and am successful.. so no idea why it doesn't work with my data..

Still not sure why the initial hillshade result looks like elephant skin ... and not something like (is it poor data quality?):

[Igor Brejc]

Hmmm.... looking again at the images you've provided, the second one (hillshading) looks a bit suspicious - it's much much too noisy compared to the original DEM. I haven't used ASTER DEM nor ArcMap, but I get the feeling the hillshading you've applied is somehow exaggerating the elevation differences, maybe you should check your hillshading parameters.

I'd also suggest trying hillshading with some other software (using the same DEM) to be able to compare results. You can also try hillshading and hypsometric tinting in Maperitive (although you won't be able to use ASTER DEM directly, just SRTM) - it never produces such noisy mess (http://maperitive.ne...ypsometric.html).

Example:

[Hans van der Maarel]

It's quite likely that your DEM is too detailed for the scale you're working on (yes, there is such a thing as being too detailed). There's some work on this subject done by Tom Patterson, Bernhard Jenny and Daniel Huffman:

Tom's comparison between manual and automatic shaded relief
Some tools by Bernhard
Daniel's take on things

Hope this helps.

[frax]

You would also have more control on all parameters by doing the hillshading in Photoshop rather than in ArcMap.

[Sab]

My preferred option is GDAL:
http://linfiniti.com...ems-using-gdal/

[DaveB]

Your original "shattered glass" result is what happens when the map units and elevation heights are in different units. Typically the x and y values are in degrees and the z value is in meters, but it could be some other combination of units if the data was projected at some point.
The way to correct that, if you're using ArcMap, is to set a z-factor when you run the hillshade tool.

Here's some info from an
Esri Mapping Center post

[rok]

Following DaveB's explanation I returned to my initial hillshading method and found out that the Z-factor setting is indeed to blame ... well, rather I am to blame for not doing my homework properly. Sab's tutorial made it more clear for me as in GDAL there are separate parameters for vertical exaggeration and ratio for converting units..

I have now the first result at hand (using more or less default hillshading settings and little noise-median filter in photoshop). Will definitely play more with this as there are tons of advice out there.. but will wait for the main data that comes over it to see which variant will suit best. Will also have to decide between stretched and classified colour ramp..
 newhillshade.png   448.32K   57 downloads  newpaintedrelief.png   411.46K   54 downloads

@Igor: will also try Maperitive, looks very cool. One Q though: if I remember correctly, you wrote somewhere on the maperitive blog that the license for ASTER GDEM data is somehow problematic. Can you please elaborate?

Thanks again to all!

[Igor Brejc]

One Q though: if I remember correctly, you wrote somewhere on the maperitive blog that the license for ASTER GDEM data is somehow problematic. Can you please elaborate?

By "problematic" I meant the license is much more restrictive than SRTM (which is in Public Domain) and overly complicated, in my view. You cannot redistribute their DEM data or any kind of derivative work that produces a lossless transformation that would allow recovery of the original data (like converting DEM to another format and putting that on some Web server).

You can, however, redistribute hillshadings, 3D renderings and similar derivative work, since it is not possible to reconstruct the original DEM data from such products.

Take a look at https://lpdaac.usgs..../aster_policies (make sure you click on the "Click here for additional GDEM redistribution information.").

One other reason why I didn't include direct ASTER DEM support in Maperitive is because you need to be registered & logged into their website in order to be able to download the data and I wanted the DEM download to run seamlessly for users (like it's in the case of SRTM data). However, if a future version of Maperitive you will be able provide your own DEM GeoTIFF tiles, so you'll be able to use ASTER.

[rok]

Thanks Igor for the legal>simple text conversion!

Looking forward to the import TIFF support in Maperitive.

[Dale A]

I'll put a vote in for Tom Patterson's shaded relief website, without doubt the best single point of reference for shaded reliefs.

Layer blending reliefs of differing resolution are a very useful way of maintaining some detail, while making sure the major watersheds, and peaks are not lost in the detail. This is particularly the case in mountainous terrain. Tom has a couple of examples on his site.

Edited by Dale A, 19 November 2011 - 01:18 AM.

[Carlos Rivas]

Thanks!
I was wrong about Z factor, but I will follow Tom guide.... he is the man!
Thanks to all