4

When speaking about a DMS or a DTM, what is posting?

And what is the difference between posting and horizontal resolution?

3

Height data can come in the form of vector or raster data. Normally "posting" or "posts" is used in conjunction with vector height data and "horizontal resolution" refers to the grid/pixel size of a raster height model. The terms 'horizontal resolution' and posting sometimes get used interchangeably, so I wouldn't get too hung up on the vector vs raster thing.

For example a DTM with a posting of 10m has an array of height posts at 10m intervals. A Raster with a horizontal resolution of 10m has a grid cell size of 10m x 10m (assuming square pixels, which does not necessarily have to be the case, but using something like GDAL_info will tell you).

In this case the raster and vector data would be effectively the same for a given area with a couple of caveats. The raster cell value can either refer to one of the corners of the cell or the center of the cell (you need to know this if you are doing interpolations). The other caveat is that, all other things being equal, the raster data will, in this case, be smaller as it only contains Z values (the X and Y values being inferred from the position of the raster origin and a column+row offset of the data value wrt the horizontal resolution).

1

Posting refers to the square cell dimensions of a given raster dataset e.g. post = 10 m means a cell size of 10 m x 10 m. What often gets mixed up though is the assumption that a DEM of a posting of 10 m has a resolution of 10 m when in fact you could represent data of a given resolution at any posting that you like.

A smaller post value will never increase your actual initial input data resolution.

Consequently, if you are provided with data of a known resolution (e.g. a Landsat tile may have a resolution of 15 m) and you resample that onto a grid of say 5 m posting (for whatever reason), to be completely up front you must state both the posting and resolution separately (i.e. posting = 5 m AND/BUT resolution = 15 m).

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