[Need Help] QGIS - Export from user-defines area

[sgtbarnes73 11]

Hello Map Makers.

I currently use Global Mapper to get my heightmaps, roads and satmap into TB as described in the “ZGM Arma 2 Map making Tutorial†guide.

I usually define a square area (for example of 4096x4096m) of the needed size and then proceed by exporting all of the different layers cropping them to this specific area.

I read in may posts some of you guys use QGIS to get all of this done, so I decided to give it a try myself and found it to be pretty a nice freeware software, but I still haven’t found a way to export data from a specific user-defined area. I made quite a few searches, but I still cannot find any specific info on this.

Is there anyone that can point out the needed steps to do so or some existing tutorial?

Thanks ahead for your appreciated response.

:)

[HellfireLx 10]

Hi,

Sorry i have no answer yet, but I have the same question, so I hope you don't mind that I join the thread. I have no experience with other GIS software, except QGIS and my terrain building experience is quite recent, but I achieved to import/create all the needed layers (heightmap in tif , a mask for clutters, primary and secondary road shapes and georeference an aerial picture). Now while all this overlays perfectly and intersects in an area i'd like to export, all the layers are of different initial size , eg: the height map covers the whole geographic degree N and E, the aerial picture is slightly bigger than the targeted area, etc. The different layers also come with different spatial reference systems , which are all nicely managed into the project reference system by QGIS. I want to dedicate some time this weekend to understand proper cutting and exporting, but if somebody already has been through this with QGIS Id be thankful for a little help. I would also like to know how to export all the layers with the proper georeference set for import in terrainbuilder (easting:200000, northings:0 in utm 31N).

I'd like also to thank all the people who wrote the get going tutorials : jakerod, zgm, bushlurkers and co.

I have the feeling that QGIS could be a good free one stop shop for preparing all the layers in real terrain building for ARMA and I'd like to be able to describe a complete workflow.

Edited April 11, 2015 by HellfireLx

[HellfireLx 10]

Hi,

I have found two things which seem to go in the right direction.

You have to create a reference layer. I mean a layer which will be used as a mask that holds the exact perimeter you are interested in. This is how I did it. There might be other ways to do it, but this works.

Edit: I found another thread where Lecholas explains how to create a square mask layer in QGIS. I followed it and then used it on my aerial layer and my height map layer.

http://forums.bistudio.com/showthread.php?131676-Convert-DEM-to-xyz-for-Visitor-3-Using-MicroDEM&p=2116808&viewfull=1#post2116808

A very very important part is the geo reference system. I had to restart my project in QGIS because I had messed up this part. Your sources will probably come in different references.

Example : My heightmap comes in WGS64 ,my satellite aerial comes in WGS64 pseudo mercator, the OSM layer in WGS64 again, those are not plane , they don't project on square meter*meter but all over the sphere in degrees. You want your project to be in UTM 31N , probably the zone your real terrain is in eg: UTM 31N.

Your project must be in UTM 31N ! All the other imported layers will be warped to that SRS during your work.

If you don't pay attention to this you are going to have problems using the mask to cut out the layers properly and converting all the layers you import into TB. It might seem to work with another UTM zone, but if you want to import real road data from OSM properly later it can be that this will become a problem when rectifying.

What I had to do for my sources. Prior I had to make sure my project was in the right UTM zone.

• Convert my height map to UTM31N Raster-Projections->Warp. Don't try to cut and convert to another file format here. It doesn't work well, just stick to converting the whole layer to the UTM geo system !
  
• Convert my imported satellite image to UTM31N. Same as with the height map.
  
• Create a mask as described by Lecholas in the link I posted. This mask has to be in the same UTM31N zone as the rest , else it will be unusable.
  
• Use Raster -> Extraction -> Clipper and use Mask Layer with your mask to cut from your satellite image and your height map. Again keep it in tiff, don't try to export to another format unless you know what you are doing or you know QGIS much better than I do, then please post feedback as I am all for shortcuts.
  
• Exporting the heigthmap in any proposed format didn't really make me happy work for me, I used L3DT to remove the georeference and resize to the wished size , in my case i wanted 1024 ( I came from 26m cell , I wanted 4m cells to have a 1024*1024 heightmap for a 4096m terrain. )
  
• Export the Satellite layer with the layer's Save As.. then chose renderer image. Then you take the resulting tiff and open it in your favorite image processing tool. There you resize to the satellite image size you need in terrain builder for your project (I had a square of 10000*10000, so i rescaled to 8192*8192 ) . I saved it as BMP, I needed to make sure no georeference data is left on that layer.
  
  It makes the import smoother in terrain builder. it let's you choose the parameters at import. I your data contains georeference which is not in the zone you chose in TB , your import simply gets rejected.
  
• With those 2 layers you can initiate your terrain properly in terrain builder. Don't forget to adjust your parameters right E: 200000 N: 0, and the matching sizes in the sampler tab.
  

That's where I am now. I already could fly over the bare map in Arma3, the overlay of the sat image and the topography match nicely. Next step, making a mask for the surface clutter and extracting the road data from the OSM layer (this one in QGIS, it's possible, I did it, but I need time to write a readable and usable tutorial!).

Hope this helped a bit.

Edited April 17, 2015 by HellfireLx
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