How to Set a Coordinate Reference System
Quick answer: A Coordinate Reference System (CRS) ties a mathematical coordinate system to the Earth via a datum. In BricsCAD, CRS, Datum, Ellipsoid, and Transformation definitions all live in one XML file, geodatabase.xml. This page covers what that structure means and how it’s organised; commands like GEOGRAPHICLOCATION, GEOIMPORT, and MAPCONNECT are what you’d actually run to apply geographic data to a drawing, though the specific steps for using them aren’t part of what’s documented here.
Coordinate System vs Coordinate Reference System
A coordinate system is just a set of mathematical rules for assigning coordinates to points, on its own, it has nothing to do with the Earth. A Coordinate Reference System (CRS) is what connects a coordinate system to the Earth, via a Datum, which models the Earth’s shape using a reference surface (an ellipsoid or spheroid).
A projected CRS is what you get by applying a map projection, a specific method and set of parameters, to a geographic CRS.
Coordinate Conversion vs Coordinate Transformation
Moving coordinates from one CRS to another falls into one of two categories:
- Coordinate conversion: no change of datum involved. The parameters are chosen directly, so there’s no error introduced.
- Coordinate transformation: the target CRS uses a different datum than the source. Because the parameters here are determined empirically (through methods like Geodetic, Seven Parameter, or Molodensky), they carry some measurement error.
Where CRS Definitions Live
All of this is defined in geodatabase.xml, found at (on Windows): <User folder>\AppData\Roaming\Bricsys\BricsCAD\<RELEASE>\en_US\Support
The file has five sections:
- CoordinateReferenceSystems
- Datums
- Ellipsoids
- Transformations
- ProjectionCodes and Methods
The CoordinateReferenceSystems Section
This is where every CRS is defined, as a CRS node with its own unique projection parameters. The most commonly relevant attributes:
| Attribute | Description |
|---|---|
epsg |
Unique EPSG database ID |
name |
Human-readable name, for example “WGS 84” |
x, y |
Orientation of the projected direction: Westing, Easting, Southing, or Northing |
proj |
The projection method, for example TMerc for Transverse Mercator, or LCC_1SP for Lambert Conic Conformal |
Lat, Lon |
Latitude/longitude of the natural origin |
FE, FN |
False easting and false northing at the origin |
SF |
Scale factor at the natural origin |
units |
The unit the projected result is in, for example “Meter” |
Each CRS node needs at least one Datum sub-node, referencing an entry in the Datums section by its epsg or alias attribute. A simplified example, for 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">
The Datums Section
A Datum combines an Earth model (an Ellipsoid) with the method used to transform it to the WGS84 model. Each Datum node needs a unique id, and can reference transformation parameters via a Transformation sub-node.
| Attribute | Description |
|---|---|
alias |
Unique datum name, for example “WGS 84” |
epsg |
Unique EPSG database ID |
id |
Unique CRS name combining projection method and datum |
pjcode |
Code for the projection method type, matching AutoCAD’s own projection codes |
The Ellipsoids Section
Each Ellipsoid node models the Earth’s shape, and needs a unique alias, a semi-major axis (a), plus at least one of flattening (f), semi-minor axis (b), or eccentricity (e).
| Attribute | Description |
|---|---|
alias |
Unique ID, for example “WGS84” |
a |
Semi-major axis length (the equatorial radius), in metres |
b |
Semi-minor axis length (equator to pole), in metres |
f |
Flattening |
e |
Eccentricity |
The Transformations Section
Defines how to convert between datums. The core attributes:
| Attribute | Description |
|---|---|
alias |
Unique ID, for example “DHDN_to_WGS84” |
source, target |
The source and target datums |
use |
The transformation method, geocentric translation, four/six/seven parameter, similarity, position vector, or coordinate frame rotation |
tX, tY, tZ |
Translation along each axis, in metres |
rX, rY, rZ |
Rotation around each axis, in degrees |
dS |
Scale difference |
Time-dependent transformations add a further set of rate-of-change parameters (for translation, rotation, and scale over time) plus a reference epoch, for cases where the transformation itself needs to account for gradual drift.
ProjectionCodes and Methods
These sections map BricsCAD’s projection and transformation definitions to AutoCAD’s own projection codes, and are what’s actually referenced inside a drawing’s AcDbGeoData object to represent its geographic location.
Frequently Asked Questions
What’s the difference between a coordinate system and a Coordinate Reference System? A coordinate system is just a mathematical framework with no connection to the Earth. A CRS ties that framework to the Earth through a datum.
Why does converting between some CRSs introduce error, but not others? It depends on whether a datum change is involved. Conversions without a datum change use fixed, error-free parameters. Transformations across different datums rely on empirically measured parameters, so some error is inherent.
Where do I actually edit or add a custom CRS? In geodatabase.xml, under the BricsCAD support folder for your installed release. This documentation covers the file’s structure; it doesn’t walk through applying a CRS to a drawing via a dialog.
What commands actually apply geographic data to a drawing? GEOGRAPHICLOCATION assigns geographic location data to a drawing, GEOIMPORT imports a drawing relative to a geographic location, and MAPCONNECT sets up a connection to a Web Map Service. Their specific usage steps aren’t covered in this source.