Raster Maps, Plans and Orthophotos

One of the digital media formats for cartographic maps is an ordinary bitmap image. A raster map is a combination of a bitmap image with a map projection and reference coordinates. With a known map projection, grid lines or corner point coordinates TransDEM can georeference such raster maps and put them them into relation with other raster maps – which may have different scale, with a DEM and with vector data (routes). TransDEM can present these different map resources at the same time at the proper positions with proper relative scale as a combination of layers. Large scale maps are called plans. Orthophotos are orbital or areal images taken from a vertical perspective with reference coordinates. Plans and orthophotos are processed like raster maps.

Note: Internally, TransDEM applies a lot of mathematics. At the surface, however, almost nothing will be revealed of the underlying algorithms. The TransDEM user interface requires minimal knowledge about map projections and coordinates, which is provided in the manual and illustrated with tutorials.

TransDEM Coordinate Systems

A precondition for processing a raster map is a known coordinate system and map projection supported by TransDEM. The native coordinate system is UTM. Additionally a number of national and international coordinate systems are supported. The majority among those are Transverse Mercator and Lambert Conic Conformal type projections but there are also a few specific ones. A few of the national systems have several zones like UTM, but most have only one zone.

Some of the TransDEM coordinate systems



Standard georeferencing in TransDEM requires three known coordinate points, preferably near the maps corners to reduce error. The coordinate system of the map must be one of those supported by TransDEM. Should the raster map carry a coordinate grid, grid line intersections are ideal points for reference coordinates.

The following example illustrates the procedure. The map is a Swiss topographic 1:25000, with Swiss Grid coordinates and grid lines. Grid line intersections used for georeferencing are marked by a circle.

Georeferencing in TransDEM this way incorporates mouse actions. The user moves the mouse cursor to a reference point – zoom is available for higher accuracy – , clicks on it and enters the coordinate values for this point. TransDEM provides guidance throughout this procedure.

If the raster map image is already aligned with the principal axes of the coordinate system prior to georeferencing, a simplified procedure can be applied.

After georeferencing TransDEM shows the result by drawing its coordinate grid onto the map image.

TransDEM accomplishes georeferencing by applying a so-called affine transformation. The three reference points determine:

  • Translation, Offset to the coordinate origin.

  • Rotation, north orientation of the map

  • Scaling, scale of the axes, which can be different for x and y

  • Shear, parallelogram-type distortion

If georeferencing is successful – and the map image is suitable for an affine transformation – the TransDEM created coordinate grid will match the on the map image with both grid lines and values.

The next step comprises the conversion to UTM. UTM is the internal TransDEM reference system. For further processing all coordinates must be UTM.

UTM conversion transforms every single pixel. It consists of the following steps:

  • Reverse affine transformation: Transformation of the bitmap into Cartesian coordinates of the selected (national) coordinate system, using the parameters determined by the reference points. This is an approximation.

  • Inverse map projection: Conversion of the Cartesian coordinates of the national coordinate system in geographic coordinates (lat/long), with the geodetic datum of the national system (reference ellipsoid). This is an exact transformation

  • Forward transformation from geographic into geocentric coordinates, with the midpoint of the national reference ellipsoid as the the origin of the geocentric system. This is exact.

  • Datum shift: Conversion of the national geodetic datum to WGS84. TransDEM applies the seven parameters Helmert transformation with translation, rotation and (uniform) scaling. This transformation is an approximation.

  • Inverse transformation of the geocentric WGS84 coordinates into geographic coordinates. Exact transformation.

  • Forward map projection of the geographicWGS84 coordinates into Cartesian UTM coordinates. Exact transformation.

In many cases the UTM conversion will lead to visible rotation of the map image. The conversion may also shrink or stretch the image in one direction, depending on the original bitmap and its coordinates.

The coordinate grid drawn by TransDEM immediately after conversion to UTM still is the national one. It will rotate (and shrink) with a the image.

TransDEM now allows to switch the coordinate system to UTM. The new grid will no longer match the grid on the map image (unless the original grid has been UTM as well) but will be aligned with the horizontal and vertical axis of the display window. Geographic coordinates (lat/long) shown in the status will also differ from those in the national coordinate system because of a different geodetic datum, which means a different geocentre.

Other Georeferencing Methods

TransDEM also supports other georeferencing methods, manual and automatic.

GeoPDF example:

Original image in Polyconic projection, a local coordinate system. Map collar identified by neat-line (semi-Transparent yellow), extracted from GeoPDF meta data.

WMS – Web Mapping Services

With version 1.2 TransDEM offers a WMS client to acquire map clippings from OGC compliant web servers, both single images and images along a path,. e.g. a railway line. Georeferencing will happen automatically. The TransDEM WMS client has many user options and comes with detailed documentation.

The screenshot depicts 1:50000 map clippings of the island of Usedom, at the Baltic Sea, Germany, with the railway line, connecting the seaside resorts with the mainland. Map data available via WMS server operated by the state surveying agency: Amt für Geoinformation Mecklenburg-Vorpommern.

Map Tile Services

TransDEM 1.3 added a Map Tile client to acquire map clippings via the map tile de-facto standard. a standard Mercator projection organized as a quad-tree for different zoom levels. Data available from various providers.

This screenshot shows Open Street Map tiles, produced by the “transport” renderer, island of Usedom again, zoom level 13 (approx. 1:120,000)

Google EarthTM Orthophotos

TransDEM allows semi-automatic georeferencing of Google Earth orthophotos. Necessary reference information is provided by a Google Earth placemark, which has to be created for each image. TransDEM reads the placemark and extracts coordinate information from it.

The screenshot shows Heringsdorf railway station on the island of Usedom as a Google Earth image in combination with the 1:25000 topographical map. TransDEM allows to zoom the display window to the user's needs.



A couple of examples follow showing some georeferenced map material in TransDEM with various coordinate systems

Germany, Gauss-Krueger

Wupper valley and the Muengsten Viaduct between Remscheid and Solingen. Map combination of 1:50000 and 1:5000. The small grey patch to the right is the large scale 1:5000 map.

Zooming in allows to view the transition between the two map scales.


Australia (New South Wales), UTM

The Blue Mountains west of Sydney provided a challenge for railway engineers. The screenshot shows the western approach between Lithgow and Mount Victoria.

The raster map images originate from a web server (meanwhile WMS is available, too). They were georeferenced individually and combined with a ASTER DEM.

These raster maps are based on vector data, recognisable when zooming in. The map shows the new and the old alignment. the old line was build with two reversals, built in Cape gauge, nowadays a preserved line “Zig Zag Railway”


Great Britain, OSGB36

This example does not depict a railway line but a historic waterway, made up from different narrow canals. Since restoration a few years ago known under the name “South Pennine Ring”. The DEM is SRTM.


Hungary, EOV

A 1:10000 topographic map and SRTM elevation data.


Switzerland, Swiss-Grid

Gotthard North Ramp around Wassen in 1:25000 with SRTM elevation data, showing also the course of the railway line as a vector overlay.


Eritrea (East Africa), S42 Pulkovo/Krassovski

Topographic map 1:500000 of Soviet origin, combined with an ASTER DEM, showing part of the 950 mm line from Asmara to Keren, currently (2005) under reconstruction. (Topographic maps produced by the Russian military surveyors exist for wide stretches of Africa and Asia under former Soviet influence. The maps are valued for their high quality.)

© 2003 - 2014 Roland Ziegler