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72 lines
2.9 KiB
ReStructuredText
72 lines
2.9 KiB
ReStructuredText
Georeferencing
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**************
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There are two parts to the georeferencing of raster datasets: the definition
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of the local, regional, or global system in which a raster's pixels are
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located; and the parameters by which pixel coordinates are transformed into
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coordinates in that system.
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Coordinate Reference System
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---------------------------
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The coordinate reference system of a dataset is accessed from its ``crs``
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attribute.
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.. code-block:: python
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>>> import rasterio
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>>> src = rasterio.open('tests/data/RGB.byte.tif')
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>>> src.crs
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{'init': u'epsg:32618'}
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Rasterio follows pyproj and uses PROJ.4 syntax in dict form as its native
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CRS syntax. If you want a WKT representation of the CRS, see the ``crs_wkt``
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attribute.
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.. code-block:: python
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>>> src.crs_wkt
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u'PROJCS["UTM Zone 18, Northern Hemisphere",GEOGCS["Unknown datum based upon the WGS 84 ellipsoid",DATUM["Not_specified_based_on_WGS_84_spheroid",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433],AUTHORITY["EPSG","4326"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-75],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","32618"]]'
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When opening a new file for writing, you may also use a CRS string as an
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argument.
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.. code-block:: python
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>>> profile = {'driver': 'GTiff', 'height': 100, 'width': 100, 'count': 1, 'dtype': rasterio.uint8}
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>>> with rasterio.open('/tmp/foo.tif', 'w', crs='EPSG:3857', **profile) as dst:
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... pass # write data to this Web Mercator projection dataset.
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Coordinate Transformation
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-------------------------
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A dataset's pixel coordinate system has its origin at the "upper left" (imagine
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it displayed on your screen). Column index increases to the right, and row
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index increases downward. The mapping of these coordinates to "world"
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coordinates in the dataset's reference system is done with an affine
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transformation matrix.
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.. code-block:: pycon
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>>> src.affine
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Affine(300.0379266750948, 0.0, 101985.0,
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0.0, -300.041782729805, 2826915.0)
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The ``Affine`` object is a named tuple with elements ``a, b, c, d, e, f``
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corresponding to the elements in the matrix equation below, in which
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a pixel's image coordinates are ``x, y`` and its world coordinates are
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``x', y'``.::
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| x' | | a b c | | x |
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| y' | = | d e f | | y |
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| 1 | | 0 0 1 | | 1 |
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The ``Affine`` class has some useful properties and methods
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described at https://github.com/sgillies/affine.
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Earlier versions of Rasterio had a ``transform`` attribute which was a 6-element
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tuple. This usage is deprecated, please see https://github.com/mapbox/rasterio/issues/86 for details.
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In Rasterio 1.0, the value of a ``transform`` attribute will be an instance
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of ``Affine`` and the ``affine`` attribute will remain as an alias.
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