Workflow from DEM to QM

[colek42]

Thank you for making this open source and your informative talk. We are looking to implement a workflow for world topo coverage using QM. What does your workflow look like going from DEM to a fully tiled set? Right now we are unsure how to best create the triangles from DEMS. Thank you again for you work.

[rolker]

I'm also in the process of using this module to create a global tileset from SRTM+ data. Here's my code: https://github.com/rolker/cesium-tools

The base SRTM data can be found here: ftp://topex.ucsd.edu/pub/srtm15_plus/

My goal is to create a few levels of topography and bathymetry worldwide, with an area of high resolution data around an area of interest.

I hope this helps!

[loicgasser]

We used commercial software to create the triangulation/mesh. But there are a few open source options out there (Grass Gis for instance).
For creating a new mesh from a regular grid you usually first create contour lines and then triangulate.
Also you should make sure you adapt your mesh density with the level of detail (around 5000 nodes max per tile) . You can either simplify existing meshes or create fewer contour lines.
My workflow involves loading several meshes in postgis and cutting right through it. You will need to re triangulate some parts at the tile edges. But that's pretty much it.
The initial project where this code was created can be found here:
https://github.com/geoadmin/3d-forge

Note that the lighting is not correct yet, as it still creates unit vector pointing in a wrong direction.
As the triangles winding order is not ensured, we need to compute both and keep the correct one.

[loicgasser]

Also to generate the geodetic profile make sure you use GlobalGeodetic, I saw this wasn t the case.
https://github.com/loicgasser/quantized-mesh-tile/blob/master/quantized_mesh_tile/global_geodetic.py

[colek42]

I have some hack-a-thon code I wrote some time ago that uses the 2nd derivative of the height to select areas of interest. https://gist.github.com/colek42/f071b3c259bf118680ce90d253812b6d

It is very messy and probably buggy as well --- it was written during a hack-a-thon.

[loicgasser]

A good tool for handling meshes is http://meshlab.sourceforge.net/

@colek42 thank you, I will have a look.

[colek42]

@loicgasser I'm working on porting your code to golang and creating an entire workflow. I should have enough to make my repo public in a couple weeks. It is not a funded project, so it will take a bit of time to get done.

[loicgasser]

Great! I would be intreseted even if my GO coding skills are suboptimal at the moment.

I am working on combining this lib with https://github.com/WoLpH/numpy-stl to create 3D stl files for 3d prints from quantized mesh tiles. If you have a suggestion, you're welcome.

[colek42]

@loicgasser I was going to use https://github.com/fogleman/simplify for simplification, stl generation from qm would probably fit well into the workflow. I am going to put a few hours in this weekend to get my tile loader working. I was going to start with HGT files from the SRTM dataset unless you have any other suggestions. I would like to stay away from GDAL due to memory leaks in the current c bindings and the complication it adds to the build process. If you want I'll give you read/write access to the repo after that portion is finished.

[loicgasser]

Hi, yes I d be interested to see that. Thanks for the pointer. SRTM is a good start. My idea concerning stl was to use existing quantized mesh tiles and combine / merge / transform them into stl files. An easy and cheap way to do it. I ll put it out there when it s ready as well.

[umeier]

I´m playing around with https://github.com/jdugge/GridToTIN (cloned to https://github.com/umeier/GridToTIN). Still a lot of work to do, but looks interesting.

[slandersson]

Hey Loic. Great work on this library.

I have files in .tif and am aiming for quantized mesh output. Any suggestions or detailed steps would be fantastic.
Should I use Grass GIS and then your library?

[loicgasser]

Yes anything that can perform the triangulation.
@umeier did you get anything out of GridToTIN?
It seems that you already did some work compared to the original version but does it work ? What is there left to do ?

[slandersson]

When you have time @loicgasser, it would be immensely useful if you could explain how to perform the triangulation in steps. It would be amazing for people like me who are new to manipulating terrain but want to work with the output. There is not much information out there in truth.

I think from there I will be able to use this library.