Ordnance Survey DEM

Lidar DSM/DTM at 2 m, 1 m, and 0.5 m resolutions.  This is so much better than the older data, you should use this if at all possible.

MICRODEM reads the United Kingdom Ordnance Survey DEM in NTF v.2.0 format, in several variants:

• 50 m post spacing, in 20 km square tiles. The file must be named according to the OS convention, such as TQ26.NTF. The first two characters reference the 100 km grid square, and the two digits the 20 km grid square.
• 10 m post spacing, in 5 km square tiles. The file must be named according to the OS convention, such as TQ53NE.NTF. The first two characters reference the 100 km grid square, the two digits the 10 km grid square, and the last two digits to the 5 km quadrant of the tile.
• 5 m post spacing, in 5 km square tiles. The file must be named according to the OS convention, which appears to follow the same rules as the 10 m DEM.
• 2 m post spacing in urban areas; I have not seen a sample of this.

The program will ignore a leading DT for 5 km tiles and a DTM for 20 km tile file names. You cannot rename the files; the registration information is in the file name. If you want descriptive names, put the files into directories.

The import assumes the grid size and spacing, and gets the DEM location from the file name. It ignores the header portion of the file, and jumps directly to the data grid. It has been tested with a number of  DEMs (at least 5 for the 50 m posting, 1 for the 10 m, and 2 for the 5 m postings.). Should there be importing problems, I would need a copy of the offending DEM to fix the code.

The British National grid uses a transverse Mercator projection, but differs from UTM:

• False easting and northing start at N49, W2 rather than the equator and a central meridian that would be W3 (or E3 for the Eastern Hemisphere part of the UK).
• Projection covers about 12 degrees of longitude, double the size of a UTM zone.
• The 100 km grid squares use a unique two letter naming convention.

For the British National Grid, see

• page A.1, The National Grid, in Ordnance Survey, Land-Form Panorama User Guide, v.2.0, 06/1997.
• page 4-8. Defense Mapping Agency, DMA TM 8358.1, Ed.1, 1998.

Landform Panorama:

• NTF and ASC formats
• https://www.ordnancesurvey.co.uk/docs/user-guides/land-form-panorama-user-guide.pdf , user guide is dated 2015
• https://www.ordnancesurvey.co.uk/business-and-government/products/land-form-panorama.html
• Collected in 1970-1980 and not updated since
• Withdrawn spring 2017, and replaced by OS Terrain 50

OS Terrain 50

• ASC and GML grids, 50 m spacing
• Also an OS Terrain 5, but that is not free.  Instead, get the Lidar DSM/DTM.

Last revised 10/24/2017