MS RFC 37: MapServer Spatial Reference Improvements and Additions¶
hobu.inc at gmail.com
warmerdam at pobox.com
To provide MapServer with the ability to set its PROJECTION information from directly from the datasource itself, in an attempt to lessen the burden related to dealing with coordinate system information on users. These improvements will be optionally available and not interfere with previous PROJECTION definition methods.
The History of Spatial References in MapServer¶
MapServer’s spatial reference support is quite anemic by many standards. While most of the data sources MapServer interacts with support describing the spatial reference of contained layers, MapServer has historically dropped the information on the floor or completely ignored it.
MapServer’s reprojection machinery keys off the fact that a LAYER’s PROJECTION is different than the MAP’s. When this is the case, MapServer reprojects the LAYER’s data to the MAP’s spatial reference during a map draw.
MapServer has historically used two different approaches for defining the spatial reference of its data – EPSG/ESRI codes in the form:
And PROJ.4-formated definitions in the form:
OGR datasources also support a form of AUTO projection type, but it is not widely advertised or regularized:
MapServer’s current spatial reference story is focussed on two things – simple description and ensuring that unnecessary data reprojection doesn’t happen. MapServer currently uses PROJ.4 directly to do its data reprojection, and this is the impetus for defining coordinate systems in PROJ.4 format. For people wanting the best performance but still requiring data reprojection, defining your spatial references in PROJ.4 format is a must.
Alternatively, the EPSG/ESRI code definition of MapServer’s spatial references allows MapServer to offload the lookup of PROJ.4 descriptions to PROJ.4 itself, with a simple file-based lookup table. This mechanism is currently a bottleneck, however, as each lookup requires trolling through a file to match the given identifier and returning the PROJ.4 definition.
This penalty was lessened in early 2009 by the addition of a caching mechanism in PROJ.4 that allows subsequent lookups to be fast.
The usability of these two mechanism can be a nightmare for users. First, most of the spatial reference descriptions that people work with are of the WKT variety – not PROJ.4. While it is straightforward to set the PROJECTION information for data with a known EPSG value, custom projections or those not generally available in the EPSG database require the user to somehow translate their WKT into PROJ.4 format and paste it into their mapfile.
Additionally, https://spatialreference.org (and https://epsg.io/) exists to help ease this pain, but it is ultimately a stopgap, and not a permanent solution to the problem. It is not practical to be downloading the spatial reference for each and every layer in a mapfile on every map draw from a website. spatialreference.org (and https://epsg.io/) does provide some conversion utilities to allow a user to paste in WKT and have it return MapServer PROJECTION blocks, but this approach still foists pain and misery on the users.
MapServer will continue to behave as before, and the user can opt-in for AUTO projection support by using the AUTO keyword in a projection object as so:
# Use the what the layer defines as the projection definition.
# This may not be available for all data sources or layer types
# (shapefile, SDE, OGR, etc.).
It is important that MapServer’s previous spatial reference definition behavior be preserved. First, drastically changing the PROJECTION definitions would mean a lot of unnecessary mapfile churn. Second, continuing to define spatial references in PROJ.4 format as before will be the most performant.
Implementation of this RFC will encompass two items:
Addition of a method to the LAYER virtual table .
Additional methods will be added for drivers to be able to convert from their native projection description type (ESRI WKT, OGC WKT, PROJ.4, etc) into PROJ.4 for setting on the projectionObj.
PROJECTION will support an explicit AUTO keyword
Virtual Table Method¶
To support AUTO projection definitions, drivers need to have the ability to return spatial reference information. MapServer’s layer plugin architecture provides mechanisms for interacting with MapServer layer providers, but there is currently no regularized method for returning the spatial reference information from providers. The following virtual table member is proposed:
int (*LayerGetAutoProjection)(layerObj *layer, projectionObj *projection)
The msOGCWKT2ProjectionObj method already exists, but a few more would be added to allow drivers that implement LayerGetAutoProjection to generate a projectionObj.
The following drivers will have implementations provided to support TYPE AUTO spatial reference definitions:
Backward Compatibility Issues¶
All work described in this RFC will provide optional capabilities to MapServer and no backward compatibility issues are expected.
This RFC will stand as primary documentation for the feature until such time as the methods and practices described in this document are integrated into the regular MapServer documentation framework.