| Author: | Howard Butler |
|---|---|
| Contact: | hobu.inc at gmail.com |
| Revision: | $Revision: 8278 $ |
| Date: | $Date: 2008-12-23 13:34:31 -0800 (Tue, 23 Dec 2008) $ |
| Status: | Draft |
| Id: | $Id: ms-rfc-37.txt 8278 2008-12-23 21:34:31Z hobu $ |
To provide MapServer with the ability to set its PROJECTION information from a number of sources, including 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 defintion methods.
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. OGC services also interact here, and when a spatial reference is specified as “available” using the METADATA mechanism, a client can request maps in a different spatial reference than is specified by default, which starts the reprojection machinery.
MapServer has historically used two different approaches for defining the spatial reference of its data – EPSG/ESRI codes in the form:
PROJECTION
"init=epsg:4326"
END
And proj4-formated definitions in the form:
PROJECTION
"proj=cea"
"lon_0=0"
"lat_ts=45"
"x_0=0"
"y_0=0"
"ellps=WGS84"
"units=m"
"no_defs"
END
A third, and rather unknown option is available exclusively to WMS – the EPSG AUTO definition, where MapServer attempts to determine the spatial reference from the data itself. This method currently only works for OGR and GDAL data sources, and it is only available when GDAL/OGR is linked into MapServer.
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 proj4 directly to do its data reprojection, and this is the impetus for defining coordinate systems in proj4 format. For people wanting the best performance but still requiring data reprojection, defining your spatial references in proj4 format is a must.
Alternatively, the EPSG/ESRI code definition of MapServer’s spatial references allows MapServer to offload the lookup of proj4 descriptions to proj4 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 proj4 definition.
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 proj4. 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 proj4 format and paste it into their mapfile.
Additionally, this author has developed a website, http://spatialreference.org, to 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 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.
Two keywords would be added to the PROJECTION block to trigger the new behavior. Without these keywords, MapServer will continue to behave as before, assuming the inputted projection is either an EPSG code or proj4 definition.
PROJECTION
TYPE TYPEENUM
VALUE "A definition"
END
The following TYPE enumerations would be supported:
VALUE is then defined as a spatial reference definition in the form denoted by the enumeration. See below for some examples:
# 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.).
PROJECTION
TYPE NATIVE
END
# Use a proj4 definition (this is essentially the same as usual except
# defined in a different way)
PROJECTION
TYPE PROJ4
VALUE "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs"
END
# Use an EPSG code
PROJECTION
TYPE EPSG
VALUE "4326"
END
# Read the definition from a file
PROJECTION
TYPE FILE
VALUE "../myfile.prj"
END
# Use an OGC WKT definition (escaping may be required)
PROJECTION
TYPE OGCWKT
VALUE 'GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]'
END
# Use an ESRI WKT definition (escaping may be required)
PROJECTION
TYPE ESRIWKT
VALUE 'GEOGCS["GCS_WGS_1984",
DATUM["D_WGS_1984",
SPHEROID["WGS_1984",6378137,298.257223563]],
PRIMEM["Greenwich",0],
UNIT["Degree",0.017453292519943295]]'
END
It is important that MapServer’s previous spatial reference definition behavior be preserved. First, drastically changing the PROJECTION defintions would mean a lot of unnecessary mapfile churn. Second, continuing to define spatial references in proj4 format as before will be the most performant.
Implementation of this RFC will encompass four items:
The following TYPE enumerations will be added to support SRS definition types:
enum MS_SRS_TYPE {MS_SRS_NATIVE, MS_SRS_PROJ4, MS_SRS_EPSG, MS_SRS_FILE, MS_SRS_OGCWKT, MS_SRS_ESRIWKT}
The MapServer projectionObj struct will have two additional members added:
char* definition
int type
These members will have their data set during mapfile parsing, and if they are not specified in the user’s PROJECTION block, they will have their values set to ‘’ and MS_SRS_PROJ4 respectively. When an empty definition of type proj4 is encountered, MapServer will default to exhibiting the previous definition behavior.
To support the additional (optional) members specified in [1] for TYPE NATIVE, 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 (*LayerGetSpatialReference)(layerObj *layer, char* definition, int type)
A default implementation for msLayerGetSpatialReference will be provide for drivers that do not define a method. It will return an empty definition of type MS_SRS_PROJ4. When MapServer encounters this, it will default to using the previous PROJECTION definition behavior.
The following drivers will have implementations provided to support TYPE NATIVE spatial reference definitions:
Four methods will be added to the SWIG MapScript projectionObj to support manipulating spatial reference information defined by this RFC.
char* getDefintion()
int getType()
int setDefintion(char* definition)
int setType(int type)
mapserver.h
mapfile.c
mapscript/swiginc/projection.i
maplayer.c
mapproject.h
mapproject.c
mapsde.c
mapogr.cpp
mapraster.c
mappostgis.c
.
.
.
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.