TITLE:  Clarification of WKT format
AUTHOR:         Name:   Adam Gawne-Cain
                Address:        Cadcorp Ltd

                Phone:
                FAX:
                Email:  adam@cadcorpdev.co.uk
DATE:   12 July 1999
CATEGORY:       Simple Features Revision Proposal

1. Background

The Simple Features specifications describe how to encode common spatial reference systems in a Well-Known Text format (WKT. The grammar of WKT is defined precisely, and there is a long list of projections that should be supported. But there is no lookup table to say what textual values should be used for the various projections. Without common textual values, different systems will not be able to communicate their spatial reference systems, even though they are using OpenGIS interfaces. This will hamper interoperability. This document proposes textual values for common projections, and lists the parameters that are expected for each of these projections.
The model of spatial reference systems used by Simple Features closely follows the model used by the European Petroleum Survey Group (EPSG. EPSG maintain a database of common spatial reference systems. The EPSG codes for spatial reference systems are used in the GeoTIFF standard, which indicates their wide acceptance. For this reason, we also show some of the differences between Simple Features names and EPSG names.

2. References

Web link for EPSG spatial reference system tables:www.epsg.org

--1-- 3. Proposal

3.1 Text values for Projections

The following textual values are proposed for projections. (The EPSG codes are also shown to clarify which projections are being referred to.

EPSG CodeWKT text value9801Lambert_Conformal_Conic_1SP9802Lambert_Conformal_Conic_2SP9803Lambert_Conformal_Conic_2SP_Belgium9804Mercator_1SP9805Mercator_2SP9806Cassini_Soldner9807Transverse_Mercator9808Transverse_Mercator_South_Orientated9809Oblique_Stereographic9810Polar_Stereographic9811New_Zealand_Map_Grid9812Hotline_Oblique_Mercator9813Laborde_Oblique_Mercator9814Swiss_Oblique_Cylindrical9815Oblique_Mercator9816Tunisia_Mining_Grid

Text values for Projection Parameters

In WKT format, each projection will have the following parameters: Lambert_Conformal_Conic_1SP latitude_of_origin central_meridian     scale_factor
false_easting false_northing
Lambert_Conformal_Conic_2SP standard_parallel_1     standard_parallel_2
latitude_of_origin central_meridian false_easting     false_northing
Lambert_Conformal_Conic_2SP_Belgium     standard_parallel_1
standard_parallel_2     latitude_of_origin
central_meridian false_easting false_northing Mercator_1SP     latitude_of_origin
central_meridian scale_factor     false_easting
false_northing Mercator_2SP
standard_parallel_1 central_meridian false_easting     false_northing
Cassini_Soldner
latitude_of_origin central_meridian false_easting false_northing Transverse_Mercator latitude_of_origin central_meridian     scale_factor
false_easting false_northing
Transverse_Mercator_South_Orientated latitude_of_origin     central_meridian
scale_factor false_easting     false_northing
Oblique_Stereographic
latitude_of_origin central_meridian scale_factor false_easting false_northing
Polar_Stereographic latitude_of_origin     central_meridian
scale_factor false_easting     false_northing
New_Zealand_Map_Grid
latitude_of_origin central_meridian false_easting     false_northing
Hotline_Oblique_Mercator
latitude_of_center longitude_of_center azimuth rectified_grid_angle scale_factor false_easting     false_northing
Laborde_Oblique_Mercator     latitude_of_center     longitude_of_center
azimuth rectified_grid_angle     scale_factor
false_easting false_northing
Swiss_Oblique_Cylindrical latitude_of_origin     central_meridian
false_easting false_northing
Oblique_Mercator    latitude_of_center
longitude_of_center     azimuth rectified_grid_angle     scale_factor
false_easting false_northing Tunisia_Mining_Grid latitude_of_origin central_meridian false_easting false_northing

3.2 Sample WKT strings

PROJCS[1999Voirol 1875 / Nord Algerie (ancienne1999, GEOGCS[1999Voirol 18751999, DATUM[1999Voirol 18751999, SPHEROID[1999Clarke 1880 (IGN1999,6378249.2,293.466021293627]], PRIMEM[1999Greenwich1999,0], UNIT[1999grad1999,1.5707963267949e-002]], PROJECTION[1999Lambert_Conformal_Conic_1SP1999], PARAMETER[1999latitude_of_origin1999,40], PARAMETER[1999central_meridian1999,3], PARAMETER[1999scale_factor1999,0.999625544], PARAMETER[1999false_easting1999,500000], PARAMETER[1999false_northing1999,300000], UNIT[1999metre1999,1]] PROJCS[1999NAD27 / Arkansas North1999, GEOGCS[1999NAD271999, DATUM[1999North American Datum 19271999, SPHEROID[1999Clarke 18661999,6378206.4,294.978698213898]], PRIMEM[1999Greenwich1999,0], UNIT[1999DDD.MMSSsss1999,1.74532925199433e-002]], PROJECTION[1999Lambert_Conformal_Conic_2SP1999], PARAMETER[1999standard_parallel_11999,36.14], PARAMETER[1999standard_parallel_21999,34.56], PARAMETER[1999latitude_of_origin1999,34.2], PARAMETER[1999central_meridian1999,-92], PARAMETER[1999false_easting1999,2000000], PARAMETER[1999false_northing1999,0], UNIT[1999US survey foot1999,0.304800609601219]] PROJCS[1999Belge 1950 (Brussels / Belge Lambert 501999, GEOGCS[1999Belge 1950 (Brussels1999, DATUM[1999Reseau National Belge 19501999, SPHEROID[1999International 19241999,6378388,297.000000000005]], PRIMEM[1999Brussels1999,4.220471], UNIT[1999DDD.MMSSsss1999,1.74532925199433e-002]], PROJECTION[1999Lambert_Conformal_Conic_2SP1999], PARAMETER[1999standard_parallel_11999,49.5], PARAMETER[1999standard_parallel_21999,51.1], PARAMETER[1999latitude_of_origin1999,90], PARAMETER[1999central_meridian1999,0], PARAMETER[1999false_easting1999,150000], PARAMETER[1999false_northing1999,5400000], UNIT[1999metre1999,1]] PROJCS[1999Makassar (Jakarta / NEIEZ1999, GEOGCS[1999Makassar (Jakarta1999, DATUM[1999Makassar1999, SPHEROID[1999Bessel 18411999,6377397.155,299.152810000006]], PRIMEM[1999Jakarta1999,106.482779], UNIT[1999degree1999,1.74532925199433e-002]], PROJECTION[1999Mercator_1SP1999], PARAMETER[1999latitude_of_origin1999,0], PARAMETER[1999central_meridian1999,110], PARAMETER[1999scale_factor1999,0.997], PARAMETER[1999false_easting1999,3900000], PARAMETER[1999false_northing1999,900000], UNIT[1999metre1999,1]] PROJCS[1999Kertau / Singapore Grid1999, GEOGCS[1999Kertau1999, DATUM[1999Kertau1999, SPHEROID[1999Everest 1830 Modified1999,6377304.063,300.801700000004]], PRIMEM[1999Greenwich1999,0], UNIT[1999DDD.MMSSsss1999,1.74532925199433e-002]], PROJECTION[1999Cassini_Soldner1999], PARAMETER[1999latitude_of_origin1999,1.1715528], PARAMETER[1999central_meridian1999,103.5110808], PARAMETER[1999false_easting1999,30000], PARAMETER[1999false_northing1999,30000], UNIT[1999metre1999,1]] PROJCS[1999NAD83 / UTM zone 10N1999, GEOGCS[1999NAD831999, DATUM[1999North American Datum 19831999, SPHEROID[1999GRS 19801999,6378137,298.257222101004]], PRIMEM[1999Greenwich1999,0], UNIT[1999degree1999,1.74532925199433e-002]], PROJECTION[1999Transverse_Mercator1999], PARAMETER[1999latitude_of_origin1999,0], PARAMETER[1999central_meridian1999,-123], PARAMETER[1999scale_factor1999,0.9996], PARAMETER[1999false_easting1999,500000], PARAMETER[1999false_northing1999,0], UNIT[1999metre1999,1]] PROJCS[1999South African Coordinate System zone 151999, GEOGCS[1999Cape1999, DATUM[1999Cape1999, SPHEROID[1999Clarke 1880 (Arc1999,6378249.145,293.46631]], PRIMEM[1999Greenwich1999,0], UNIT[1999degree1999,1.74532925199433e-002]], PROJECTION[1999Transverse_Mercator1999], PARAMETER[1999latitude_of_origin1999,0], PARAMETER[1999central_meridian1999,15], PARAMETER[1999scale_factor1999,1], PARAMETER[1999false_easting1999,0], PARAMETER[1999false_northing1999,0], UNIT[1999metre1999,1]] PROJCS[1999RD / Netherlands Old1999, GEOGCS[1999Amersfoort1999, DATUM[1999Amersfoort1999, SPHEROID[1999Bessel 18411999,6377397.155,299.152810000006]], PRIMEM[1999Greenwich1999,0], UNIT[1999DDD.MMSSsss1999,1.74532925199433e-002]], PROJECTION[1999Oblique_Stereographic1999], PARAMETER[1999latitude_of_origin1999,52.0922178], PARAMETER[1999central_meridian1999,5.23155], PARAMETER[1999scale_factor1999,0.9999079], PARAMETER[1999false_easting1999,0], PARAMETER[1999false_northing1999,0], UNIT[1999metre1999,1]] PROJCS[1999WGS 84 / UPS North1999, GEOGCS[1999WGS 841999, DATUM[1999World Geodetic System 19841999, SPHEROID[1999WGS 841999,6378137,298.257223562997]], PRIMEM[1999Greenwich1999,0], UNIT[1999degree1999,1.74532925199433e-002]], PROJECTION[1999Polar_Stereographic1999], PARAMETER[1999latitude_of_origin1999,90], PARAMETER[1999central_meridian1999,0], PARAMETER[1999scale_factor1999,0.994], PARAMETER[1999false_easting1999,2000000], PARAMETER[1999false_northing1999,2000000], UNIT[1999metre1999,1]] PROJCS[1999GD49 / New Zealand Map Grid1999, GEOGCS[1999GD491999, DATUM[1999New Zealand Geodetic Datum 19491999, SPHEROID[1999International 19241999,6378388,297.000000000005]], PRIMEM[1999Greenwich1999,0], UNIT[1999degree1999,1.74532925199433e-002]], PROJECTION[1999New_Zealand_Map_Grid1999], PARAMETER[1999latitude_of_origin1999,-41], PARAMETER[1999central_meridian1999,173], PARAMETER[1999false_easting1999,2510000], PARAMETER[1999false_northing1999,6023150], UNIT[1999metre1999,1]] PROJCS[1999NAD27 / Alaska zone 11999, GEOGCS[1999NAD271999, DATUM[1999North American Datum 19271999, SPHEROID[1999Clarke 18661999,6378206.4,294.978698213898]], PRIMEM[1999Greenwich1999,0], UNIT[1999DDD.MMSSsss1999,1.74532925199433e-002]], PROJECTION[1999Oblique_Mercator1999], PARAMETER[1999latitude_of_center1999,57], PARAMETER[1999longitude_of_center1999,-133.4], PARAMETER[1999azimuth1999,323.07483685], PARAMETER[1999rectified_grid_angle1999,323.07483685], PARAMETER[1999scale_factor1999,0.9999], PARAMETER[1999false_easting1999,16404166.67], PARAMETER[1999false_northing1999,-16404166.67], UNIT[1999US survey foot1999,0.304800609601219]] PROJCS[1999Tananarive (Paris / Laborde Grid1999, GEOGCS[1999Tananarive (Paris1999, DATUM[1999Tananarive 19251999, SPHEROID[1999International 19241999,6378388,297.000000000005]], PRIMEM[1999Paris1999,2.5969213], UNIT[1999grad1999,1.5707963267949e-002]], PROJECTION[1999Oblique_Mercator1999], PARAMETER[1999latitude_of_center1999,-21], PARAMETER[1999longitude_of_center1999,49], PARAMETER[1999azimuth1999,21], PARAMETER[1999rectified_grid_angle1999,21], PARAMETER[1999scale_factor1999,0.9995], PARAMETER[1999false_easting1999,400000], PARAMETER[1999false_northing1999,800000], UNIT[1999metre1999,1]] PROJCS[1999Carthage (Paris / Tunisia Mining Grid1999, GEOGCS[1999Carthage (Paris1999, DATUM[1999Carthage1999, SPHEROID[1999Clarke 1880 (IGN1999,6378249.2,293.466021293627]], PRIMEM[1999Paris1999,2.5969213], UNIT[1999grad1999,1.5707963267949e-002]], PROJECTION[1999Tunisia_Mining_Grid1999], PARAMETER[1999latitude_of_origin1999,38.81924], PARAMETER[1999central_meridian1999,7.84257], PARAMETER[1999false_easting1999,270.66603], PARAMETER[1999false_northing1999,581.95123], UNIT[1999kilometre1999,1000]]

Differences from EPSG

The Spatial Reference model described in the OpenGIS Simple Features specification is so similar to the EPSG model, that it is worth pointing out the differences. In this document, we have followed the conventions set out in the existing Simple Features specification. However, in future we could choose to change the existing SF spec to conform to the EPSG model where sensible.

SF has more projections than EPSG

The Simple Features specification lists many more projections than EPSG. SF lists about 44 projections, whereas EPSG only lists about 16. This is largely because EPSG only lists projections that are used for projected coordinate systems. The SF specification also lists projections that are used for presenting graphics.

Projection Names

EPSG uses the name “Lambert Conic Conformal”, whereas SF uses the name “Lambert Conformal Conic”.

Parameter Names

The text values used for projection parameters in Simple Features WKT are significantly different from the parameter names used in by EPSG. EPSG termOpenGIS SF termAngle from Rectified to Skew Gridrectified_grid_angleAzimuth of initial lineazimuthEasting at false originfalse_eastingEasting at projection centrefalse_eastingFalse eastingfalse_eastingFalse northingfalse_northingLatitude of false originlatitude_of_originLatitude of first standard parallelstandard_parallel_1Latitude of natural originlatitude_of_originLatitude of originlatitude_of_originLatitude of projection centrelatitude_of_centerLatitude of second standard parallelstandard_parallel_2Longitude of false origincentral_meridianLongitude of natural origincentral_meridianLongitude of origincentral_meridianLongitude of projection centrelongitude_of_centerNorthing at false originfalse_northingNorthing at projection centrefalse_northingScale factor at natural originscale_factorScale factor on initial linescale_factor

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