[GIS] Manipulating Azimuthal Equidistant Projections in QGIS

coordinate systemqgis

I have a shapefile of world countries that is projected as Azimuthal Equidistant with Chicago, USA as the center. I would like to change the projection such that Edinburgh, UK is the center. One would think that this should be as simple as changing 2 numbers in the coordinate system parameters…

The problem that I seem to be experiencing is that QGIS makes it difficult if not impossible to view the complete proj4 code of a given shapefile's coordinate system parameters.

This is what I have tried so far:

Added shapefile of world countries projected as Azimuthal Equidistant with Chicago as the center
Opened properties, metadata tab, copy proj4 parameter code
Went to settings, custom projection, paste in proj4 code, changed the lat/lon from that of Chicago to that of Edinburgh
Saved shapefile of world countries, specified my new custom projection, added new file to map
My new file looks like a map of Pangaea, with the continents all smushed together.

I think there must be other parameters aside from those shown in the proj4 code of the metadata tab. The only thing I'm changing in the proj4 code is the center of the map, but the change I'm seeing is much more than that. My observation suggests that more than one independent variable is changing.

I would be happy to send my shapefile to anyone who would like to play with it.

Thanks,

Daniel Wolf
Environmental/Geospatial Enthusiast

Best Answer

Using a simple proj string:

+proj=aeqd +lat_0=51 +lon_0=7

I get this picture from Natural Earth world boundaries, and a 10-degree-grid that should span from -150° to 150°E:

aeqd on a WGS84 ellipsoid

It looks like everything outside of lon_0 +/-90° gets truncated, and parts of the southern hemisphere bended as well.

EDIT

According to this ticket: http://trac.osgeo.org/proj/ticket/211 proj has a problem with reprojecting aeqd data on an ellipsoid.

A simple solution: use a sphere (and not the default ellipsoid) by defining a radius:

+proj=aeqd  +R=6371000 +lat_0=51 +lon_0=7

This gives me the following picture:

aeqd on a sphere

Grid lines are densified to show nice curves.

Related Solutions

[GIS] Projecting Azimuthal Equidistant global map into circle using QGIS

To get am Azimuthal Equidistant map, follow my answer here:

Manipulating Azimuthal Equidistant Projections in QGIS

But that gives you an image of the whole world. You have to crop that picture to a circle around the point of origin in a second step.

But I'm not sure if you do not rather want an orthographic map:

http://en.wikipedia.org/wiki/Orthographic_projection_%28cartography%29

http://commons.wikimedia.org/wiki/Grey-green_orthographic_projections_maps

This is what you get with

+proj=ortho  +R=6371000 +lat_0=0 +lon_0=7

enter image description here

[GIS] Losing polygons after projecting map in QGIS

The simple reason why QGIS can not draw these polygons is because some of the vertices are on the backside of the globe, and QGIS can not draw a closed polygon with the rest. The GDAL ogr2ogr function in the background is programmed to kick off the whole feature when such an error occurs.

So you have two solutions:

Solution 1: Clip your polygons to the hemisphere

You need Numerical Digitize and CAD Tools plugin for that.

Set the project CRS to ortho projection
create a new point layer in the same projection
with the Numerical Digitize plugin, create the following points:

(should also work with Add delimited text layer)

enable snapping on the point layer
with CAD Tools, select the first three points and create an arc from them
do the same for points 3 - 4 - 1 for the southern hemisphere
change project CRS to WGS84
Save the CAD layer as WGS84 and add that to the canvas
Save your boundary layer also as WGS84 and add it to the canvas
convert the CAD lines to a new polygon layer
remove all except the polygon layers
clip the world boundaries to the CAD polygon layer
set project CRS back to ortho

You should get this picture: world vector in ortho projection

Solution 2: Convert your vector data to raster

in Settings, disable on-the-fly-reprojection
Save your boundary layer as WGS84 and add that to the canvas
Use Raster -> Convert -> Rasterize on the WGS84 layer select x=360 and y=170 as dimensions for the raster (or n*360 and n*170)
Use Raster -> Projection -> Reproject to the ortho projection under another name and add it to the canvas. You will get some errors, but the rest of the raster will do. Don't bother that you see a grey rectangle
Rightclick on the raster layer -> Properties, Style tab
choose pseudo colours
in transparency tab, choose 0 for 100% transparency
Remove the first raster
set project CRS to ortho

you should see the follwing picture: enter image description here

The same method works with rasters, as you can see in my avatar picture ;-)