TransDEM Forum

What is the vertical resolution of TransDEM?

I have just come across the info about the 2m LIDAR data for England, as detailed here http://forum.transdem.de/viewtopic.php?f=2&t=392.

This looks very useful, especially as it is designed for purposes such as flood defenses, it (the DTM version) omits buildings but includes embankments and cuttings. The course of railway lines is very clearly shown where they are in cuttings or embankments.

The documentation here http://environment.data.gov.uk/ds/docs/ ... FAQ_v3.pdf states the vertical accuracy is +/- 4cm. Trainz Surveyor will allow track spline point height modifications in increments of 0.01m (1cm).

I considered this could be used to align tracks exactly in a vertical direction, as well as their horizontal alignment. As the 5m Trainz grid may loose definition of embankments or cuttings, I intend to use the TranzDEM window and "mouse-over" an embankment, the height is then detailed in the lower edge of the map window.

But this value is only displayed in whole metres.

As the resolution of the Lidar data is 0.04m, is the resolution of TransDEM less than 1m, and if so can this be displayed somewhere to enable exact track heights and gradients to be modelled?

Thanks
David

David,

internally, the vertical resolution is single precision floating point (32bit), i.e. 7 decimal digits. That is the same as in most binary file formats for higher resolution DEM. The Environment Agency LIDAR are delivered in an ASCII format, but will also be converted to single precision floating point. (For lower resolution DEMs there are quite a few other DEM file formats that store the data in integral metres, e.g. the ubiquitous SRTM .hgt.)

This full vertical resolution will be applied when exporting to Trainz, but you are right, of course, horizontal resolution has to be reduced to 5m. (TransDEM uses bilinear interpolation here.)

The status line always shows elevation as integer values at the moment. But I'll have a look at it. I am currently working on the last remaining tasks for TransDEM 2.6, final testing, updating the manual, etc., but this may still fit in. I guess it should be dependent on the DEM data source. If not delivered in floating point, centimetres don't make that much sense.

In this Transdem screenshot, the railway embankments are very clear, the colour range showing the height. The main line, from Trent Sth Jn (the bottom edge on the right), can be seen to fall slightly to Trent Jn (up to the right), with the High Level Goods line climbing to cross the main line.

The West Curve also climbs slightly to Sheet Stores Jn, (right of centre of the image), where the main lines split to the Derby (left edge) and Chellaston (bottom edge on left side) lines. The northern side of the triangle, actually the East Curve, also climbs from right to left. "Mouse-over" the image in Transdem shows Trent Jn at 31m, and Sheet Stores Jn at 34m. In the screenshot the mouse was placed over Sheet Stores Jn, 34m shows in the bottom of the window.

In Trainz, the area looks like this, looking SE towards Sheet Stores Jn, with the Derby & Chellaston lines converging, then the East & West Curves diverging again. Also the road over the canal (on the former course of the railway) can be seen rising to the bridge gap then falling again.



Using the Get Height function in Surveyor tabs, the height of the compass comes out at 33.64m.

It seems to me as if there may be almost too much detail in the 2m Lidar data, it may be easier to build on the older Landform Panorama data, but use the 2m Lidar data in Transdem to get the exact height of the trackbed at certain points, by "mouse-over"ing in Transdem at those points.

More experimenting needed here.

I have added elevation centimetres in the status bar label. The threshold for displaying the fractional part is a horizontal DEM raster of less than 10m. For DEMs with a point spacing of 10m and above nothing will change.

geophil wrote:I have added elevation centimetres in the status bar label. The threshold for displaying the fractional part is a horizontal DEM raster of less than 10m. For DEMs with a point spacing of 10m and above nothing will change.

Thanks. I can now be certain that my route has its tracks at the correct elevation, rather than just guessing...