Q on translation between (x, y) and (lat, lon) coordinates

[yaofuzhou]

May I kindly ask how are the (x, y) coordinates relate to the (lat, lon) coordinates? I understand that the 3D models supplied by Blender GIS has a known (lat, lon) coordinate for the origin, and the points of the 3D model is expressed in the (x, y, z) coordinates. How may I translate them to the (lat, lon) coordinates? In case approximations are involved. My area of interest has the dimension of up to 200 km.

Appreciate any help that may improve my understanding of this issue!

[domlysz]

Well that's what we call map projection
https://en.wikipedia.org/wiki/Map_projection
https://github.com/domlysz/BlenderGIS/wiki/Georeferencing-management

[baswein]

With a GIS program re-projection is not too intensive in fact QGIS does it on the fly without you even noticing anything. QGIS has great tools to import and process OSM data. I have heard that there are rounding issues with blender and large distances but really am not familiar with that. What exactly are you trying to do?

[yaofuzhou]

Thank you again for telling me about QGIS @baswein.

I have been using pyproj to do the reproduction, which was somehow the slowest part of my program. My task, to begin with, is to get the terrain/skyline of a city of ~100km across in a customized format with vertices in (lat, lon, z) coordinates (EPSG:4326). The (naive) way I am doing it is:

1. Use BlenderGIS to import a manageable portion of the city.
2. Export to ASCII .stl format, in which vertices are in (x, y, z) coordinates (EPSG:3857).
3. Use Python with pyproj to reproject EPSG:3857 to EPSG:4326.
4. Repeat above for the next portion.
5. Rounding issue arises when I try to put adjacent regions with slight overlaps together. I have limited the number of decimal places in the (lat, lon) coordinates and have been able to remove duplicate data in the overlap.
6. In addition, as mentioned, pyproj takes over a day for the reprojections…

I deeply appreciate you telling me that this can be done in a much more efficient way!

[baswein]

Unfortunately I am probably as naive as you but I don't think the export to shapefile and QGIS will really work for you because you would still need to figure out how to get the shapefile back to stl. Have you looked at BlenderOSM?

[yaofuzhou]

Not yet @baswein Sounds like another item on my to-know-about list.

In case it helps, the only reason I was working with ASCII .stl is that I can humanly read the vertices, and I can easily extract the (x,y,z) coordinates. This is no longer necessary.

In any case, the seemingly promising approach for me is to get terrain/skyline directly in the (lat, lon) format from OSM or so. Then other than the final rendering I can simply stick to (lat, lon).

Appreciate your time and hints!

[baswein]

If you want to send me an email - bas at rdgland com - we can continue this conversation by email in case we are bugging domlysz