[OSM-dev] Plotting data on an equirectangular map, distorted for sphere projection

Martin Raifer tyr.asd at gmail.com
Thu Jul 18 08:12:21 UTC 2013


Just a wild guess (didn't try), but what if you rendered your map in an  
equal area projection (e.g. Mollweide) and reprojected the resulting  
raster image - in a second step - to equirectangular (for example using  
GDAL)? That could result in something more towards your objective...


Christopher Stevens <chris at christopherstevens.cc>:

> Hello Dane,
> Thanks for taking a look. So far I can produce an EPSG:4326 image nicely,
> your comment boosted that setup (thanks!). This one is quick and ugly,  
> but
> should work for illustration purposes:
> http://www.flickr.com/photos/8139783@N08/9309514743/
> Now to add some basic large labels:
> http://www.flickr.com/photos/8139783@N08/9309518499/in/photostream/
> If I then project this on a sphere (virtual one in this example, but  
> looks
> the same in real projection), the geography placement looks great and  
> lines
> up well with other datasets. The styling gets odd with labels and lines
> getting thinner horizontally.
> http://www.flickr.com/photos/8139783@N08/9309514621/in/photostream/ -  
> side
> view
> http://www.flickr.com/photos/8139783@N08/9309514685/in/photostream/ -  
> pole
> view, the presenter often turns the globe this way for audience
> Let me know if that helps illustrate the challenge better.
> Thanks again,
> Chris
> On Wed, Jul 17, 2013 at 11:41 AM, Dane Springmeyer <dane at dbsgeo.com>  
> wrote:
>> Chris,
>> It would be helpful to explain what you've tried already. Mapnik  
>> supports
>> proj4 and proj4 understands quite a few projections. I'm no expert in
>> equirectangular projections but
>> https://en.wikipedia.org/wiki/Equirectangular_projection states that  
>> this
>> is basically a term for the well know set of lon/lat based geographic
>> coordinate systems.
>> So, can you try simply changing your Mapnik Map `srs` value to EPSG:4326
>> or a specific Equidistant cylindrical projection like
>> http://spatialreference.org/ref/epsg/3786/
>> The specific Mapnik Map XML syntax can be found by following the link  
>> for
>> "Mapnik XML" like:
>> http://spatialreference.org/ref/epsg/3786/mapnik/
>> Dane
>> On Jul 17, 2013, at 9:41 AM, Christopher Stevens <
>> chris at christopherstevens.cc> wrote:
>> Hello gurus,
>> I'm thinking that I may need to find some alternative data visualization
>> software to make this work, but I love mapnik and thought it would be  
>> good
>> to start here first.
>> I'm hoping to create a basic themed map of different types of data and
>> plot it on a full-Earth image (I can figure this part out, starter  
>> examples
>> cover this as well). The challenge is that I'm then projecting this on  
>> Science
>> On a  
>> Sphere<http://www.spacefoundation.org/visit/northrop-grumman-science-center-featuring-science-sphere/science-sphere%C2%AE>,
>> which requires equirectangular images that distort more towards the  
>> poles.
>> Here's a quick example of something I made in a 3D program to show  
>> latitude
>> and longitude lines (note how the lines and labels get fuzzy and  
>> stretched
>> near the poles).
>> http://www.flickr.com/photos/8139783@N08/9305646391/
>> Is there a way to add this type of distortion in mapnik? I'm guessing  
>> not
>> as mapnik is optimized for other uses (but who knows). I'm guessing  
>> that it
>> would "work" as is, but labels would get distorted (there are
>> work-arounds), and lines would get very thin near the poles.
>> Thanks for the feedback in advance. Let me know if I can answer any
>> questions.
>> Best regards,
>> Chris

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