[OSGeo-Discuss] Call for help: GDAL and txt2shp.py

[Kjell Are Refsvik]

On 7. juli. 2008, at 19.19, Dane Springmeyer wrote:

> Hi Kjell,
> ...
> You are not alone. Projected coordinate systems can confuse  
> beginners and experts alike.

:-)

Well, given my completely newbie status as a GIS/Geodata user, I am  
very grateful for your patience, explaining these things to me.

> My understanding is that you have gps data and you are not sure what  
> coordinate system it is in. You ran Matt Perry's script to convert  
> the text-based coordinates into a shapefile format, but you still  
> need to assign a coordinate system.

I understand. WGS84 and UTM 32V worked fine for the data from Norway.

> ...
> First, you only need to assign the projection AFTER you run the  
> txt2shp.py script.

OK. I understand.

> ...
> Second, I can see now (from your image links) that your data was  
> collected in Lebanon, which means that the link I directed you to  
> will not provide a reference to the correct UTM zone.
>
> There are two possible UTM zones for Lebanon: UTM 36N or UTM 37N.
>
> WGS 84 / UTM zone 36N
> http://spatialreference.org/ref/epsg/32636/
>
> WGS 84 / UTM zone 37N
> http://spatialreference.org/ref/epsg/32637/

36N seemed to have done thr trick:
PROJCS["WGS_1984_UTM_Zone_36N",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",33],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["Meter",1]]
> You can check for yourself by downloading a world borders shapefile (http://thematicmapping.org/downloads/world_borders.php 
> ) and a world UTM zone shapefile (http://earth-info.nga.mil/GandG/coordsys/gislayers/zips/mgrs6x8.zip 
> ), both in the WGS 84/EPSG 4326 coordinate system.
>
> If you have more GPS data from other countries I highly recommend  
> getting familiar with UTM zones.

No. Only have data from Norway and Lebanon so far, but looking more  
closely into UTM seems like a good idea, especially in order to to get  
to grips with paper maps.

http://www.ia-stud.hiof.no/~kjellare/misc/lebanon_wgs84_UTM36N.png

> ...
> If you try to load data in different coordinate systems into the  
> same QGIS project, QGIS  will not (by default) make an effort to  
> 'reproject-on-the-fly' when rendering, unless you explicitly set the  
> Qgis project to do so. So even if you assign the correct UTM zone to  
> your points, if you open them in Qgis along with a base layer in WGS  
> 84 projection, they will not line up. You need to go to SETTINGS >  
> PROJECT PROPERTIES > PROJECTION > ENABLE ON THE FLY PROJECTION.

I figured that out, playing around with the Projection settings.

> Or use the command line program ogr2ogr to reproject your shapefile  
> to WGS84/EPSG 4326 from your assumed UTM projection. That command  
> would look like:
>
> $ ogr2ogr -s_srs EPSG:32636 -t_srs EPSG:4326  
> lebanon_points_wgs84.shp lebanon_points_utm36N.shp

Splendid. I will have a go at that.

> Then, if the assumed source projection was correct, your new  
> shapefile should line up with other data in WGS 84.

> ...
> Yes, they are GUI applications... but don't you just need to  
> automate the processing of your GPS data?

No. In addition to use a unix shell-script to harvest and prepare  
geodata from my images using exiftool and some basic text formatting  
commands in unix, I also need to prepare the necessary shapefiles and  
hopefully find some way of (using shell-scripts) atomatically export  
png files to make up the map.

A quick sketch (using a screengrabbed OpenStreetMap) here:
http://www.ia-stud.hiof.no/~kjellare/misc/programming_goal.png

> If you want to create one PNG raster then QGIS or uDIG are the ideal  
> tools to layout your png map. Just open all your shapefiles (you can  
> even merge them all into one shapefile with ogr2ogr), label them,  
> then zoom to each group and export a PNG file....
>
> If you truly want to automate the creation of  *many* PNG graphics  
> then you'll need to look into scripting a mapping toolkit like  
> Mapserver or Mapnik.

Ouch. I see now that what looked like a way out of this (shp2img) is  
not accepting a shapefile as a input.

> Dane

Any idea on the complexity in getting the above scripting to work?

Sincerely,

Kjell Are Refsvik
Norway

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