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Stage 16 — Which choices move the map?

The Introduction ends with a question: which choices change the answer, and by how much? This stage answers it with one comparison. Every choice we tested is run against the main map (D8 connectivity at 25 m or 100 m, LIST 2021 land-use pressure averaged over each named system, slope multiplier 0.5 to 1) and scored with one measure, the rank correlation over the mapped karst cells: 1 means the same ordering of cells, lower means a bigger change.

Choices are grouped under the three tests of the project:

GroupChoices
ScaleRouting on a 2 m DEM (Mole Creek only); the 100 m grid against 25 m (Mole Creek only)
DataDEA 2020 land cover instead of LIST land use
MethodThe same four ingredients added instead of multiplied; six inputs added as a weighted sum; a slope ceiling of 10 degrees; COP’s Scenario 2 slope direction; exposure removed from susceptibility (susceptibility = intensity only)

Details of each test are in Stages 8, 9, 13 and 14; this stage only brings the numbers together on one measure.

16.1 Mole Creek (25 m)

Mole Creek: 412,523 karst cells
Method: six ingredients added       0.549
Method: slope, flatter = higher     0.825
Scale: 100 m DEM                    0.894
Method: exposure removed            0.907
Data: DEA land cover                0.979
Method: four ingredients added      0.979
Method: slope, 10 degree ceiling    0.992
Scale: 2 m DEM                      0.994

16.2 Statewide (100 m)

Tasmania: 410,000 karst cells
Method: four ingredients added      0.704
Method: six ingredients added       0.821
Data: DEA land cover                0.843
Method: slope, flatter = higher     0.955
Method: exposure removed            0.963
Method: slope, 10 degree ceiling    0.987

16.3 The chart

                                  Mole Creek (25 m)  Tasmania (100 m)   group                    choice
Scale: 2 m DEM                                0.994               NaN   Scale                   2 m DEM
Scale: 100 m DEM                              0.894               NaN   Scale                 100 m DEM
Data: DEA land cover                          0.979             0.843    Data            DEA land cover
Method: four ingredients added                0.979             0.704  Method    Four ingredients added
Method: six ingredients added                 0.549             0.821  Method     Six ingredients added
Method: slope, flatter = higher               0.825             0.955  Method   Slope, flatter = higher
Method: slope, 10 degree ceiling              0.992             0.987  Method  Slope, 10 degree ceiling
Method: exposure removed                      0.907             0.963  Method          Exposure removed
<Figure size 1050x640 with 1 Axes>
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Result: most choices leave the ordering of the karst nearly unchanged, and a few change it a lot.

  • Little change (rank correlation 0.98 or above): routing on a 2 m DEM (0.99 at Mole Creek), the slope ceiling (0.99 at both), and, at Mole Creek only, DEA land cover in place of LIST (0.98) and adding the same four ingredients instead of multiplying (0.98).

  • Moderate (0.70 to 0.96): the 100 m grid against 25 m at Mole Creek (0.89), removing exposure (0.91 at Mole Creek, 0.96 statewide), COP’s slope direction (0.83 at Mole Creek, 0.96 statewide) and, statewide, DEA land cover (0.84), the six-input weighted sum (0.82) and adding the same four ingredients (0.70).

  • Large (below 0.6): the six-input weighted sum at Mole Creek (0.55).

The land-cover year is not tested here: it is a change to track on the final map (Stage 9 shows the pattern stays stable year to year while the level moves). The method choices matter most, meaning which ingredients are added or multiplied, and the slope rule. Adding the same four ingredients keeps the Mole Creek ordering (0.98), while adding six different inputs does not (0.55), so the extra inputs change the map there. Exposure changes the map about as much as the 100 m grid does, and it is part of the main model because it improves the springs check (Stage 10). Mole Creek moves more than the state under several choices because it is a small, closely mapped area where one or two systems set the ordering.