by Dennis Collin
When producing site drawings, particularly for infrastructure and larger scale projects, there is a need to be able to reference satellite imagery to a site plan, pipe or other large service network. This is possible by using AutoCAD’s geographic location tool under the Insert tab on the Ribbon menu. It is worth noting this function is also available in AutoCAD LT.
Fig 1. Mapping overlay information can be accessed from the Insert tab within AutoCAD/LT.
In the 2024 version, this can be provided either from a map from the online mapping service, or from a Google KML or KMZ format file. When using the Geographic location function, coordinate information is required in a global coordination format in terms of degrees latitude and longitude from the Greenwich Meridian. However , users in AutoCAD typically with be expressing Coordinates in a global cartesian format, using an Ordnance Survey grid as the datum. This is usually what the WCS (or World Coordinate System) is set to.
Fig 2. Use the ID command to obtain cartesian coordinates of a known location.
Therefore, these coordinates will need to be translated into the correct format. There are many such translation tools available to users, one example is the service provided by “Grid Reference Finder” this will convert a variety of coordinate system notations into a format accepted by the location function command.
Fig 3. Use the Grid Reference Finder service to translate cartesian coordinates into degrees latitudes/longitude.
Once transformed, copy and paste the latitude and longitude result into an application like Notepad for easy pasting into AutoCAD’s location dialogue. When running the location command, paste the geographic coordinates into the location dialogue and ensure that the drawing units are consistent with the drawing (Millimetres or Metres). Place a marker onto the displayed marker in an aerial photograph preview image and confirm.
Fig 3. Dropping a location marker onto the preview image.
A dialogue will then ask what Coordinate System to use.
Fig 4. Selecting a suitable GIS Coordinate System for the UK.
For UK projects I tend to use the GIS mapping system, OSTN15.NationalGrid, which should be precise enough for most needs. Elsewhere in Europe and the world other systems should be considered due to different coordinate systems, origin alignments and irregularities of the Earth geoid. For more information on systems, and the joys of mapping an irregular oblate geoid, read an Ordnance Survey blog on the different Grid systems and mapping translations here.
Fig 5. Pipe network drawing with satellite image underlay attached!
Confirm the location by snapping to the corresponding AutoCAD reference geometry and confirm the north angle which is 90 degrees typically in a drawing. The imagery should display as an underlay and dependent on the reference system used a best fit obtained.
The image underlay can be captured to a viewport, plotted and issued on a sheet in the same way as ordinary image attachments.