Tutorial: georeferenced drawings

In this tutorial you set up a drawing in real-world coordinates: you give the drawing a coordinate system, insert a georeferenced orthophoto and overlay the parcels of a SHP file, which LinceCAD converts to the drawing's system if it comes in another one.

You need two sets of files for the same area, like those published by official mapping agencies:

An orthophoto in GeoTIFF, or in TIFF, JPG or PNG with its world file (.tfw, .jgw, .pgw or .wld) next to it. Here it is called ortofoto.tif.
A SHP with its companion files (.shx, .dbf and, preferably, .prj). Here it is called parcelas.shp.

The example uses ETRS89 / UTM zone 30N (EPSG:25830), the official system in much of mainland Spain; use the one for your area. In UTM, one drawing unit is one metre.

1. The drawing's coordinate system

Start a drawing with NEW.
Enter GEOGRAPHICLOCATION (or GEO). The Geographic Location dialog opens and, at the same time, the command line asks:

Enter EPSG code of the coordinate system or [Remove/List] <…>:

Type 25830 and Enter (or choose the system from the list in the dialog and click OK). The command line confirms the chosen system.

This is the target system: everything imported with its own system is converted to it. Type L (List) to see the available systems: ETRS89, WGS84 and ED50 in zones 28 to 31, REGCAN95, Web Mercator (3857) and geographic coordinates. The AutoCAD code, such as ETRS89.UTM-30N, is accepted too.

The drawing is assumed to be, or to be going to be, in coordinates of that system: GEOGRAPHICLOCATION does not move what is already drawn.

2. The orthophoto

Create an Orthophoto layer and make it current, so that you can turn it off or lock it later.
Enter GEOIMAGE.

Enter image file name to attach or [Browse] <Browse>:

Press Enter (Browse) and choose ortofoto.tif.
The image is placed at its coordinates, with its pixel size and rotation, read from the world file or the GeoTIFF tags. The command line says which georeferencing file it used and which system the image is in.
Double-click the mouse wheel to see the whole image.

If the image states its coordinate system and it differs from the drawing's, it is reprojected on insertion. If it does not, it is inserted as it is: its coordinates must already be in the drawing's system.

3. The SHP parcels

Create a Parcels layer, with a colour that stands out on the photo (for example 2, yellow), and make it current.
Enter MAPIMPORT (or SHP2CAD). The Import SHP, KML or GeoJSON dialog opens.
In File click Browse… and choose parcelas.shp. The dialog shows the Contents: how many features and how many fields it has.
Layer from field: leave none so that everything goes to the current layer, or choose a field of the table (for example, the land use or type field): each feature goes to a layer named after the value of that field, created if it does not exist.
Block for the points: only matters if the file has points. With (point object), each point is a POINT object; with a block of the drawing, each point is that block, and its attributes take the values of the fields with the same name.
File coordinate system: the one stated in the .prj appears. If the file has no .prj, choose its system here; with (no reprojection) the coordinates are imported as they are.
Drawing coordinate system shows EPSG:25830. Click OK.

The parcels are drawn as closed polylines (polygon holes too) over the orthophoto. If the SHP was in another system, for example zone 29 or geographic coordinates, it has already been converted. The table data of each feature is kept with it, as extended data.

The same can be done on the command line, for example in a script:

SHP, KML, KMZ or GeoJSON file:

Field for the layer (…) or [None] <None>:

Block name for the points <point object>:

4. Checking coordinates

Enter GEOCOORD.

Specify point or [System] <EPSG:…>:

Click a parcel corner: the command line shows its drawing coordinates and its WGS84 latitude and longitude. With S (System) you choose another target system, for example 4326 or 23030 (ED50).
With the drawing georeferenced, clicking the coordinates on the status bar toggles them with the latitude and longitude of the cursor.

5. Saving and sharing

Save as DWG: the coordinate system is saved the way AutoCAD saves it, and AutoCAD recognizes it when it opens the drawing (GEOGRAPHICLOCATION).
The image is not stored inside the DWG: the drawing points to the file. Send the orthophoto and its world file along with the drawing.
MAPEXPORT writes the selected objects to SHP, KML, KMZ or GeoJSON, with the table data they came with. KML and GeoJSON are in WGS84: a KMZ opens directly in map viewers.

Notes

Image formats: TIFF and GeoTIFF, JPG, PNG, BMP, GIF and WebP. ECW and JPEG 2000 are not read: convert them to GeoTIFF first.
If the image has no georeferencing, place it with GEOREFIMAGE, giving two or three points on the image and their real position.
So that the orthophoto does not cover the parcels, send it to the back with DRAWORDER. IMAGEADJUST changes its brightness and contrast, and TRANSPARENCY lets what is underneath show through its transparent pixels.
With the ED50 datum, the conversion to ETRS89 uses the official IGN grid (SPED2ETV2.gsb) if it is in the program's rejillas folder (centimetre accuracy); otherwise, a 7-parameter transformation (≈ 1.5 m).