Geolocation and GIS

A georeferenced drawing has its coordinates in a real-world reference system (for example, ETRS89 / UTM 30N): every point of the drawing is a point on the ground. This way orthophotos, maps, GIS files and GPS data can be overlaid without moving them by hand.

Coordinate system of the drawing

GEOGRAPHICLOCATION assigns a coordinate system to the drawing. Type its EPSG code (25830, 4326...) or the Autodesk code (ETRS89.UTM-30N), or choose it in the dialog box; the List option shows the available systems and Remove deletes it. The drawing must already be in coordinates of that system: GEOGRAPHICLOCATION moves nothing, it only states which system the drawing is in. It is saved as in AutoCAD, which recognizes it.

The full list (over 300) is in the GEOGRAPHICLOCATION dialog box, grouped by country or region and searchable by EPSG code, name or country, and in the List command-line option. The parameters of each projection are those of EPSG (checked against PROJ to the millimeter).

Datum shift accuracy. Between ETRS89, WGS84 and the GRS80-based frames (SIRGAS, MAGNA-SIRGAS, POSGAR, RGF93, ETRF2000-PL, SWEREF99…) there is no datum shift: the transformation is exact (as in PROJ, ETRS89 = WGS84). Older datums use the best EPSG transformation that needs no grid, with its nominal accuracy: about 1 m for Amersfoort, BD72, LUREF, CH1903+, GGRS87, HD72 and Nord Sahara 1959; 1.2 m for Egypt 1907; 1.5 m for MGI; 2 m for OSGB36 (the official OSTN15 grid is not included: it can reach 3–4 m); 3 m for DHDN (the former West Germany one) and Stereo 70; 4 m for Monte Mario; 7 m for Merchich and 14 m for Carthage. For accurate work, draw in the country's modern system (ETRS89, SIRGAS…). ED50 transformations use the official NTv2 grid of the Spanish IGN if it is installed (in the rejillas folder next to the program or in AutoCAD's); otherwise a 7-parameter transformation with an error of about 1.5 m, valid only in Spain.

Geographic coordinates

• With a coordinate system assigned, clicking the coordinates on the status bar adds the cursor's latitude and longitude.
• GEOCOORD shows a point in WGS84 (latitude and longitude) or, with the System option, in any other system of the catalog.

GIS import and export

MAPIMPORT brings SHP (with its DBF), KML, KMZ and GeoJSON files into the drawing:

• points, lines and polygons (holes as closed polylines);
• reprojected to the drawing's system according to the SHP's .prj (KML and GeoJSON are always in WGS84);
• the attributes of each feature are stored on the object as extended data;
• the layer can come from a field, and points can be a block whose attributes take the values of the fields with the same name.

MAPEXPORT saves the selection to SHP, KML, KMZ or GeoJSON; the format comes from the file extension. SHP is written in the drawing's system, with its .prj and one file per geometry type. KML, KMZ and GeoJSON are written in WGS84 and need the drawing to have a coordinate system.

Georeferenced images

• GEOIMAGE inserts an image at its real coordinates from its world file (.tfw, .jgw, .pgw, .bpw, .wld...) or its GeoTIFF tags. If the image carries its coordinate system (.prj or GeoTIFF) and it is not the drawing's, the image is reprojected.
• GEOREFIMAGE places an image without georeferencing using control points: for each one, specify the point on the image and its real position. With two points the image is moved, rotated and scaled exactly; with three it is fitted by least squares.

Large orthophotos are loaded in tiles (see Raster images). ECW, MrSID and JPEG 2000 cannot be read: CONVERTIMAGE turns them into GeoTIFF with GDAL (installed separately), keeping the size and the georeferencing, and attaches the GeoTIFF in the same place in the drawing.

Tips

• Use meters as drawing units (UNITS) when working in UTM.
• With UTM coordinates the numbers are large (X ≈ 400,000, Y ≈ 4,500,000): use ZOOMEXTENTS to find the drawing.

Online maps

MAPSERVICE loads maps from WMS, WFS, WMTS and XYZ services as a background or as vectors. The Spanish servers are built in; those of other countries are in Map servers by country.

See also

Raster images · Coordinates · Tutorial: georeferenced plan · Map servers by country