dirty-reprojectors
v0.0.2
Published
make different projections
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Dirty Reprojectors
Quick and dirty re-projections to trick your web maps out of web mercator.
Install
npm install -g dirty-reprojectors
Usage
CLI
cat input.geojson | dirty-reproject --forward PROJECTION [--reverse PROJECTION=mercator] > output.geojson
Example: to reproject some geojson so that web mapping libraries will render it looking like 'albersUsa':
cat input.geojson | dirty-reproject --forward albersUsa > output.geojson
For a list of supported projections, dirty-reproject --list
API
reproject
Reprojects the given geometry coordinate array in place, with
unprojectable points or degenerate geometries removed. If both
options.forward
and options.reverse
are supplied, then forward
is
performed first.
Parameters
options
Objectoptions.forward
(Function | string)? The forward projection to use.options.reverse
(Function | string)? The reverse projection to use.options.projections
Object? A map of named projections to use. If provided, then string values ofoptions.forward
oroptions.reverse
will be used as keys to look up the projection function inoptions.projections
. For an extensive list provided by d3-geo-projection, userequire('dirty-reprojectors/projections')
.
coordinates
Array
How it works
Take, for example:
cat input.geojson | dirty-reproject --forward albersUsa > output.geojson
What this actually does is:
- Project
input.geojson
from WGS 84 (longitude/latitude) intoalbersUsa
, with the target coordinates scaled to match the dimensions of Web Mercator. - Reverse-project the result back to WGS84 as if it had been projected with Web Mercator. So now, when your favorite web mapping library tries to project it into mercator, the geometries end up looking like they were projected using Albers.
The main catch is that if you actually look at the longitude/latitude
coordinates in output.geojson
, they are totally wrong. (There are other,
subtler catches, too, having to do with Web Mercator's limited latitude range,
varying loss of precision, and probably many other nuances I am not aware of.)
Credits
- Inspired by this trick
- All the heavy lifting here is thanks to Mike Bostock's excellent d3-geo and de-geo-projection