Coordinate reference system

A Coordinate System is a set of mathematical rules for specifying how coordinates are to be assigned to points.

The coordinate system is unrelated to the Earth. A Coordinate Reference System (CRS) is a coordinate system related to the Earth through a Datum. A Geodetic Datum is a model of the Earth that specifies a reference surface (Ellipsoid or Spheroid).

A projected coordinate reference system is the result of the application of a map projection to a geographic coordinate reference system. A map projection is a type of coordinate conversion. It uses an identified method with specific formulas and a set of parameters specific to that coordinate conversion method.

Coordinates may be changed from one CRS to another through the coordinate operation. Two types of coordinate operation may be distinguished:

  • coordinate conversion, where no change of datum is involved and the parameters are chosen and thus error free.
  • coordinate transformation, where the target CRS is based on a different datum to the source CRS. Transformation parameters are empirically determined and thus subject to measurement errors. (Geodetic, Seven parameters, Molodenksy...)

Geographic datasets are defined in geodatabase.xmlthat is placed in <User folder>\AppData\Roaming\Bricsys\BricsCAD\<RELEASE>\en_US\Support for Windows. The content of this file is composed of the 5 sections:

  1. CoordinateReferenceSystems
  2. Datums
  3. Ellipsoids
  4. Transformations
  5. ProjectionCodes and Methods

CoordinateReferenceSystems

This section is the main XML node where all Coordinate Reference Systems are defined. It has many sub-nodes called CRS, each XML node must define a unique projected coordinate reference system and the necessary projected parameters.

The supported CRS projection types are described in the ProjectionCodes and Methods sections.

CRS node attributes:

Follow Table 1 of OGP Geomatics Guidance Note No 7, part 2

Attribute name Description Units
epsg Unique EPSG database id. Integer
name Human-readable name of CRS, Example: "WGS 84 -- WGS84 - World Geodetic System 1984, used in GPS". String
codeSpace Owner of CRS. It is optional. String
x, y Orientation of projected direction. Possible values:
  • "Westing"
  • "Easting"
  • "Southing"
  • "Northing"
Directions MUST be defined.		 String
xAxis, yAxis Orientation of geographic direction. Possible values:
  • "Latitude"
  • "Longitude"
 String
Greenwich Greenwich Meridian relation. Degree
proj
Projection method of CRS. Possible values:
  • "LL", "LatLon", "LonLat" is a Geodetic Latitude-Longitude.
  • "MercA", "Mercator_1SP" is a Mercator (variant A).
  • "MercB", "Mercator_2SP" is a Mercator (variant B).
  • "MercC" is Mercator (variant C).
  • "MercSp" is a Mercator (Spherical).
  • "MercPv" is a Popular Visualisation Pseudo Mercator.
  • "TMerc" is a Transverse Mercator.
  • "TMercSO" is a Transverse Mercator South Orientated.
  • "LCC_1SP" is a Lambert Conic Conformal 1SP.
  • "LCC_2SP" is a Lambert Conic Conformal 2SP.
  • "LCC_2SP_BE" is a Lambert Conic Conformal 2SP Belgium.
 String
AngleSG Angle from Rectified to Skew Grid. Degree
Azimuth Azimuth of initial line. String
FE Easting at false origin, False easting. Float-Point value in CRS units
EC Easting at projection center. Float-Point value in CRS units
FN Northing at false origin, False northing. Float-Point value in CRS units
FC Northing at projection center. Float-Point value in CRS units
LonI The longitude of the western limit of the first zone of a Transverse Mercator zoned grid system. Degree
Lat Latitude of natural origin, Latitude of standard parallel. It depends on Projection method. Degree
Lat1 Latitude of 1st standard parallel. Degree
Lat2 Latitude of 2nd standard parallel. Degree
LatF Latitude of false origin. Degree
LatC Latitude of projection center. Degree
Lon Longitude of natural origin, Longitude of origin. Degree
LonF Longitude of false origin. Degree
LonC Longitude of projection center. Degree
SF Scale factor at natural origin. Float-Point value, unity
SFIL Scale factor on initial line. Float-Point value, unity
SFPSP Scale factor on pseudo standard parallel. Float-Point value, unity
W Zone width in longitude. Degree
zone Zoned Grid System. Degree
units Units of conversion result from geographic coordinates to projected. Example: "Meter", unit "Degree" means no conversion, Geographic Coordinate System. String
Example: common attributes for all map projections based on EPSG:31468. 
<CRS epsg="31468" codeSpace="OGP" name="DHDN / 3-degree Gauss-Kruger zone 4" y="Easting" x="Northing" 
Greenwich="0" proj="TMerc" Lon="12" Lat="0" SF="1" FE="4500000" FN="0" zone="4" units="Meter">

For each node CRS there should be at least one sub-node Datum that refers via epsg or alias attributes to sub-node Datum in the Datums section. The sub-node Datum MUST have the id attribute of a unique CRS name. At least one of the epsg code and the alias of Datum must be valid.

Attribute name Description Units
epsg Unique EPSG database id. Example: "4326". Integer
alias Unique Datum name. Example: "WGS 84". String
id Unique CRS name: combination of Projected method and Datum. Example: "WORLD-MERCATOR". It related to CS-MAP, AutoCAD® names. String
pjcode

Indicate code for the coordinate reference system projection method types. Corresponded to AutoCAD® projection codes.

For example, 3 - Transverse Mercator, 44 - UTM, 45 - Transverse Mercator of Snyder formulation and etc.

 Integer
Example: there are unique CRS definitions according to Datums inside CRS node:
<Datum epsg="6314" alias="DHDN/3" id="DHDN/3.Gauss3d-4" pjcode="3" />
<Datum epsg="6314" alias="DHDN/2" id="DHDN/2.Gauss3d-4" pjcode="3" />
<Datum epsg="6314" alias="DHDN" id="DHDN.Gauss3d-4" pjcode="3" />

Datums

This section is a main node where all Datums are stored. Datum is a combination of the Earth model (Ellipsoid or Spheroid) and the transformation method to model WGS84. A Datum node specifies common parameters and contains at least one inner sub-nodes Datum exactly one sub-node Ellipsoid. Each inner sub-node Datum should define a unique name within the id attribute and may specify transformation parameters to WGS84 model via sub-node Transformation that refers via epsg or alias attributes to a sub-node Transformation in the Transformations section.

Attribute name Description Units
alias Unique id. Example: "DHDN/3". It is related to CS-MAP, AutoCAD® names. String
epsg Unique EPSG database id. Example: "6314". Integer
name Human-readable name of Datum. Example: "Deutsches Hauptdreiecksnetz". String
codeSpace Owner of CRS. It is optional. String
Example:
<Datums>
<Datum epsg="6314" codeSpace="OGP" name="Deutsches Hauptdreiecksnetz ">
<Datum id="DHDN/3">
<Transformation epsg="1777" alias="DHDN/3_to_WGS84" />
</Datum>
<Datum id="DHDN/2" name="Deprecated - Replaced by DHDN/3">
<Transformation epsg="1777" alias="DHDN/2_to_WGS84" />
</Datum>
<Datum id="DHDN" name="Deprecated - Replaced by DHDN/2">
<Transformation epsg="1673" alias="DHDN_to_WGS84" />
</Datum>
<Ellipsoid epsg="7004" alias="BESSEL" />
</Datum>
…
</Datums>

Ellipsoids

The Ellipsoids section is the main section where models of the Earth are defined. Each Ellipsoid node should define an identification by a unique name (alias) and a semi-major axis (a) and one of the following attributes at least: flattening (f), semi-minor axis (b) or eccentricity (e).

Attribute name Description Units
alias Unique id. Example: "WGS84". It related to CS-MAP, AutoCAD® names. String
epsg Unique EPSG database id. Example: "7030". Integer
name Human-readable name of Datum, Example: "WGS 84". String
codeSpace Owner of CRS. It is optional. String
a Length of the semi-major axis of the ellipsoid, the radius of the equator. Float-Point value, strongly in Meter
b Length of the semi-minor axis of the ellipsoid, the distance along the ellipsoid axis between equator and pole. Float-Point value, strongly in Meter
f Flattening Float-Point value, unity
e Eccentricity Float-Point value, unity
Example:
<Ellipsoid epsg="7008" alias="CLRK66" name="Clarke 1866, Benoit Ratio" a="6378206.4000000004" 
b="6356583.7999999998" f="294.9786982139" e="0.0822718542" />

Transformations

This section is a main node where all transformation methods are defined. Supported Transformation methods are described in the ProjectionCodes and Methods sections.

Attribute name Description Units
alias Unique id. Example: "DHDN_to_WGS84". It related to CS-MAP, AutoCAD® names. String
epsg Unique EPSG database id. Example: "1673". Integer
codeSpace Owner of Transformation. It is optional. String
source Source Datum. String
target Target Datum. String
use Transformation method. Supported transformations using geocentric methods:
  • Geocentric translations
  • Four/Six/Seven Parameter Transformation
  • Similarity Transformation
  • Position Vector transformation
  • Coordinate Frame rotation
 String
method Methods of building of rotation matrix, if applicable, "PVT" is Position Vector Transformation, "CFR" is Coordinate Frame Rotation. String
tX X-axis translation. Meter
tY Y-axis translation. Meter
tZ Z-axis translation. Meter
rX X-axis rotation. Degree
rY Y-axis rotation. Degree
rZ Z-axis rotation. Degree
dS Scale difference. Unity
xp Coordinate 1 of evaluation point. Meter
yp Coordinate 2 of evaluation point. Meter
zp Coordinate 3 of evaluation point. Meter
dtX Rate of change of X-axis translation. Unity
dtY Rate of change of Y-axis translation. Unity
dtZ Rate of change of Z-axis translation. Unity
drX Rate of change of X-axis rotation. Unity
drY Rate of change of Y-axis rotation. Unity
drZ Rate of change of Z-axis rotation. Unity
ddS Rate of change of scale difference. Unity
t0 The reference epoch for time-dependent parameters. Float point value
fallback Alias of transformation from target datum to WGS84 datum. String
Example:
<Transformation epsg="1679" alias="Pulkovo42/2_to_WGS84" src="Pulkovo42/2" trd="WGS84" use="Param7" 
method="CFR" tx="-40.595" ty="-18.55" tz="-69.339" ds="-4.299" rx="-2.508" ry="-1.832" rz="2.611" 
accuracy="9" />

ProjectionCodes and Methods

These sections describe the ProjectionCodes and Methods that are mapped to AutoCAD® definitions of projections and transformations types. They are used to store the definitions in an XML file of coordinate reference system definition inside the AcDbGeoData object that represents a geographic location.