MS RFC 100: Support for tile index with mixed SRS in raster layers¶
even dot rouault at mines-paris dot org
Raster layers can directly connect to a GDAL dataset or through a tile index. But until now, the rasters in the same tile index should have identical SRS.
For the sake of simplicity of the administration of MapServer mapfiles, and/or to avoid unnecessary reprojections of rasters into a common SRS (which might be different from the requested SRS of a WMS or WCS request), it is desirable to have support for tile index with rasters of heterogeneous SRS.
2. Proposed solution¶
In the case where a tile index has tiles with mixed SRS, a new keyword, TILESRS «field_name», must be added in a LAYER block to specify that the field «field_name» contains the SRS of each tile. The value of that field must be a SRS expressed as WKT, PROJ.4 string, or EPSG:xxxx code (actually any string that is parsed by OSRSetFromUserInput()). If, for a tile, the field is empty, an attempt will be made to fetch the SRS from the tile raster directly with GDALGetProjectionRef().
Note that it is absolutely necesserary that the geometry (extent) of each tile is expressed in a common SRS (the one declared in the PROJECTION block of the LAYER block), so that spatial queries in the tileindex can work.
Such tileindex files that have tiles with mixed SRS can be generated with an updated version of the GDAL gdaltindex utility, by using the -t_srs and -src_srs_name options.
gdaltindex -t_srs EPSG:4326 -src_srs_name src_srs mytileindex.shp *.tif
Those tile index can be used for example in WMS or WCS layers. For WCS layers, a few metadata fields must be specified in the LAYER definition, so as to define a «virtual dataset» coverage (see WCS Server). A new GDAL python sample script, wcs_virtds_params.py, will be added to ease the writing of such layers.
3. Implementation Details¶
3.1 Files affected¶
The main changes are in mapraster.c. Note that a code refactoring has been done to factor identical code found in msDrawRasterLayerLow() from mapraster.c and msRasterQueryByRect() from maprasterquery.c.
mapcopy.c : deal with layerObj.tilesrs
mapfile.c : deal with layerObj.tilesrs and TILESRS
mapfile.dtd : deal with TILESRS
mapfile.h : add TILESRS constant
maplexer.c : regenerated from maplexer.l
maplexer.l : deal with TILESRS
mapogr.cpp : generates error if TILESRS is used in a vector layer (unsupported)
mapraster.c : core of changes to deal with layerObj.tilesrs; code rafactoring
mapraster.h : new file to declare common functions between mapraster.c and maprasterquery.c
maprasterquery.c : refactored to share common code with mapraster.c
mapserver.h : add tilesrs member in layerObj struct
mapshape.c : generates error if TILESRS is used in a vector layer (unsupported). add capability to get projection info from .prj file when PROJECTION = AUTO
3.2 Backwards Compatibility Issues¶
None expected, new functionality.
3.3 MapScript changes¶
3.4 Performance implications¶
Finding tiles that match the request window will be as fast as for tile index with same SRS. On-the-fly reprojection will only occur for tiles that intersect the request window. So the performance should be similar to current situations where a tile index with tiles in SRS=srs1 is requested with SRS=srs2.
This RFC only implements TILESRS support for raster layers. Support in vector layers is out of scope.
4. Bug ID¶
6. Voting history¶
Vote passed with +1 from ThomasB, StephenW, MichaelS, TamasS, StephanM, DanielM, JeffM and SteveL