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.
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
UTM: a transverse Mercator projection (Universal Transverse Mercator), dividing the Globe into 60 zones of 6° degree longitude each, based on geodetic datum WGS84 (World Geodetic System 1984) UTM with geodetic datum NAD83 (North America), GDA94 (Australia) or ETRS89 (Europe) can be treated as equivalent to WGS84 for TransDEM purposes.
Europe: UTM / ED50. An older UTM system.
British and Irish Grids. Transverse Mercator projections. One zone each.
Benelux grids. A stereographic projection for the Netherlands, a Lambert projection for Belgium and a transverse Mercator projection for Luxembourg. One zone each.
French grids: Older and newer Lambert projections. The older system has four zones, the newer system only one.
German grid: transverse Mercator (Gauss/Krueger), four zones.
Swiss and Austrian grids: An oblique Mercator projection for Switzerland and transverse Mercator (Gauss/Krueger) for Austria. One zone for the Swiss system, three for Austria.
Italian grid: transverse Mercator, two zones.
Czech and Slovak grid: A Lambert derivative, one zone for both Czechia and Slovakia.
Hungarian grid:a combination of a spherical and cylindrical projection, one zone.
Eastern European and Russian grid: Two variants of the Pulkovo/Krassovski projections (transverse Mercator), several zones.
North America: UTM with NAD27 datum, the older system.
Australian grids: transverse Mercator and Lambert projections for New South Wales, Victoria and South Australia. Older and newer systems. Lambert projections with one zone each.
New Zealand grids: older NZMG (specific) and new NZTM (transverse Mercator).
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.
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TransDEM accomplishes georeferencing by applying a so-called affine transformation. The three reference points determine:
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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.
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UTM conversion transforms every single pixel. It consists of the following steps:
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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.
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.
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
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.
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”
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.
A 1:10000 topographic map and SRTM elevation data.
Gotthard North Ramp around Wassen in 1:25000 with SRTM elevation data, showing also the course of the railway line as a vector overlay.
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 - 2007 Roland Ziegler
