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Vertical resolution of Transdem? https://forum.transdem.de/viewtopic.php?f=2&t=397 |
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Author: | davidbird [ 10 Dec 2015 22:45 ] |
Post subject: | Vertical resolution of Transdem? |
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/Environment_Agency_LIDAR_Open_Data_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 |
Author: | geophil [ 11 Dec 2015 10:38 ] |
Post subject: | Re: Vertical resolution of Transdem? |
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. |
Author: | geophil [ 13 Dec 2015 18:47 ] |
Post subject: | Re: Vertical resolution of Transdem? |
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. |
Author: | davidbird [ 13 Dec 2015 22:36 ] |
Post subject: | Re: Vertical resolution of Transdem? |
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... |
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