Ordnance Survey is committed to open data formats. The grid data is supplied as ASCII with GML, to enable their use in either format. The GML file does not contain any spatial height data as this data has been provided as an ‘external data block’, that is, the ASCII grid file. The GML file effectively provides metadata (such as location, grid spacing and the vertical reference system). It also contains spatial reference information in a software-independent form.
Currently, common software packages do not support it in this form, but the ASCII grid for OS Terrain 5 grid can be used alone.
ASCII grid is a generic, text-based DTM format that was originally developed by Esri, and it is sometimes referred to as ArcInfo ASCII grid or ArcGrid ASCII. This data can be read by most standard GIS software without additional translation.
The term ‘data structure’ refers to the organisation and sequence of the records in the data file and not to the geographic topology of the data.
The data is specified as a raster grid, with the height values being calculated as a mean of the heights across the whole 5m pixel. To represent this in ASCII grid format, the z values are presented 2.5 m from the north-west corner of the tile (to provide the pixel-centre). The data is presented in rows reading fromwest to east creating a row of 1,000 values. The next row will begin 2.5m from the western edge 7.5m south of the northern tile edge (under the first value) and again progressing at 5m intervals to the east. Each tile contains 1,000 rows with each row comprising 1,000 pixels. The header provides the coordinates of the south-west corner (xllcorner and yllcorner) to ensure that a GIS places the data correctly.
The height values are presented in the standard ASCII grid format as a series of real values and are given to the nearest 0.01m.
The structure of the layers provided with the ASCII grid and GML download is outlined in the following table:
The following is the data header found in the first six rows of the ASCII grid file:
| Layer name | Layer description |
|---|---|
<tile name>.asc
Esri ASCII grid data.
<tile name>.gml
OGC file for the ASCII grid to enable the data to be loaded as GML format.
<tile name>.prj
File containing the spatial reference system in a format defined by Esri.
<tile name>.asc.aux.xml
A file that provides parameters to enable default styling in Esri applications. This ensures that the shading is consistent across the data but allows you to apply your own choice of colour ramp.
Metadata_<tile name>.xml
A metadata file for grid data, providing information on the flying date and so on.