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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