Using Raster and Vector Height Data in WindFarm

I will use the UK Ordnance Survey Terrain 50, Terrain 5 and Panorama height data as examples, but the same principles apply to data from other parts of the World.

First of all what do I mean by "raster" and "vector" (my terminology, but it has been used elsewhere). The Panorama SD60 20km tile supplied with WindFarm is what I would call a vector file. Across the 20km region there are 401 height points at 0m, 50m, 100m etc from the origin. The Terrain 50 data, supplied in 10km tiles, is what I would call a raster data set, and there are 200 cells across the 10km region, with the height specified at the centre of each cell. The 200 height points are therefore at 25m, 75m, 125m etc from the origin.

So basically if you think of a 50m grid covering a 10km region the height data points are at the corners of the grid cells for a vector data set and at the centre of each grid cell for a raster data set.

The Terrain 5 data is in 5km tiles with 5m grid spacing. As a result the height data are specified at 2.5m, 7.5m, 12.5m etc.

Why does this matter? If you have one data set, covering the whole region that you need, then it does not matter much which you use, except that for the raster data you do not have height data points extending to the edge of the region – they stop half a cell short.

Problems may arise when you have a group of data sets covering a region. With most vector data sets there is usually no problem because you can interpolate across the whole region specified in the tile. There is usually a one-point overlap at each edge with an adjacent tile. However, if you need the height at the edge of a raster tile you have to find and examine the adjacent tile in order to interpolate a height value.

How does this affect WindFarm? If a group of Terrain 50 tile are specified for a region and the map region is set to a multiple of 50m the line of points between the tiles will not be interpolated and will be set to zero. As an example 4 tiles (SD60, SD61, SD70 and SD71) are needed to cover the 20km region of the Panorama 20km SD60 tile. Setting a 20km wind farm site map region to 360000, 400000 origin with a 50m grid spacing means that the South / North line of points at 370000 East and the West / East line at 410000 North will not be defined. You will get a warning of missing points if you try this in a project.

What is the solution? If you specify a group of raster tiles in a project then make sure that the origin coincides with the centre of a cell and the spacing matches the cell size. So for the Terrain 50 example above you would need to specify 360025, 400025 as the origin and 50m as the grid spacing.

A second, less error prone method, is to join the raster tiles first using the Grid Viewer. Having converted your raster tiles (usually Arc/Info ASCII grid format *.asc) to GTH files following the information below use the File / New menu option and specify all the tiles needed to cover your region. When you click the Initialise button the region will be correctly set to the centre of cell values. The resulting height data file can be used directly in your project.

A minor debate on the new Terrain 50 data has been taking place in the OS blog:


Converting Raster Data to WindFarm’s GTH Format

After initialising the map region on the Output page of the Data Conversion module the region will be set to the point locations (360025,400025 to 369975,409975 for the 10km SD60 tile of Terrain 50 data). Retain these values and switch off the “Force coordinates to be a multiple of the grid spacing” check box. Continue with the conversion and the GTH file will have the same points defined as the original data set.

When converting a Terrain 5 data set then because WindFarm uses 1m resolution the region limits will be rounded by 0.5m to 3m, 8m, 13m etc (360003,400003 to 364998,404998 for the 5km SD60SW tile of Terrain 50 data). Retain these values for the map region, switch off the “Force coordinates to be a multiple of the grid spacing” check box and continue with the conversion.

When you join multiple Terrain 50 or Terrain 5 tiles in the Grid Viewer these offsets (25m for Terrain 50 and 3m for Terrain 5) will be set when the map region is initialised. Do not change these values unless you want a different map region, in which case make sure you keep the region aligned with the offsets.

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