Surface Statistics Algorithm
Surface Statistics Algorithm Update – 8.3.3
The surface statistics algorithm has been updated to reduce edge affects when calculating the surface coverage. In previous versions of FM8 and in FM7, the algorithm was using surrounding grid cells to calculate the sloping area between cells. This method has its draw backs one being that cells on the edge were not being included in the calculation because their neighbors don’t have any data. To eliminate these edge affects and include all of the valid cells the algorithm has been updated to explicitly count cells. For the surface area calculation if a cell has valid data its cell area is added to the total surface area. For volume calculations this cell area is multiplied by the depth attributed to that cell and added to the total volume.
Surface Statistics and Re-projection
When comparing statistics calculated from the same surface in two different coordinate systems, the values will likely not be the same. The re-projection process takes the bounds of the input surface projects them to the output coordinate system, then “back projects” the location of each cell centre in the resulting surface to the input surface and then uses this location to query the z-value from the original surface and write it to the new location in the re-projected surface. In doing this it is very likely that the surface coverage will change and therefore the surface statistics will also change.
Surface Statistics and Surface Difference
As of version 8.4.0 there have been updates to the surface difference operation to increase the number of options a user has for selecting what data they would like to include in the surface difference result.
Previously (before 8.4.0), the data selection options were:
- only data above (delta z < 0)
- only data below (delta z > 0)
- all data
Additional options available in 8.4.0 and newer:
- data above and at (delta z <= 0)
- data below and at (delta z >= 0)
When comparing statistics computed on a surface difference result, we can expect that the surface area and volume values calculated with the “only data above” option selected and the values calculated with the “data below and at” option selected would add up to equal the values calculated when the “all data” option is selected.
IE: only data above (delta z < 0) area + data below and at (delta z >= 0) area = all data area
IE: only data above (delta z < 0) volume + data below and at (delta z >= 0) volume= all data volume
IE: only data below (delta z > 0) area + data above and at (delta z <= 0) area = all data area
IE: only data below (delta z > 0) volume + data above and at (delta z <= 0) volume= all data volume