Valid -> Invalid polygons can occur

[andrewharvey]

reducing coordinate precision of all coordinates in a polygon can unwittingly create invalid polygons. I'm not sure if it's the place of this program to care about that but it would be nice if no invalid polygons were written out.

[jonjardine]

Very useful tool, however as mentioned above invalid polygons can be produced, where two points become identical due to the rounding / clipping process. MySQL's geometry calculations will report invalid polygons in these cases.

An example is below:

{
   "type":"Feature",
   "properties":{
      "gml_id":"SSSI_SCOTLAND.902",
      "NAME":"Ness of Cullivoe",
      "PA_CODE":1217,
      "STATUS":"Current",
      "SITE_HA":10.36,
      "TYPE":"GEOLOGICAL",
      "MPA_NET":"N",
      "GEO_LOC":"MIXED"
   },
   "geometry":{
      "type":"Polygon",
      "coordinates":[
         [
            [
               -0.994949049275153,
               60.702545894931433
            ],
            [
               -0.994947414191508,
               60.7025395884272
            ],
            [
               -0.994951217317521,
               60.7025351274714
            ],
            [
               -0.99496215976439,
               60.70253611639918
            ],
            [
               -0.994969392320168,
               60.702539753233921
            ],
            [
               -0.994969280238507,
               60.702543343972998
            ],
            [
               -0.994959982580452,
               60.7025477637278
            ],
            [
               -0.994949049275153,
               60.702545894931433
            ]
         ]
      ]
   }
}

After conversion to 5 decimal places, this produces:

{
      "type":"Feature",
      "properties":{
         "STATUS":"Current",
         "NAME":"Ness of Cullivoe",
         "TYPE":"GEOLOGICAL",
         "SITE_HA":10.36,I'v
         "gml_id":"SSSI_SCOTLAND.902",
         "MPA_NET":"N",
         "GEO_LOC":"MIXED",
         "PA_CODE":1217
      }
   },
   {
      "geometry":{
         "type":"Polygon",
         "coordinates":[
            [
               [
                  -0.99465,
                  60.7026
               ],
               [
                  -0.99467,
                  60.70259
               ],
               [
                  -0.99468,
                  60.70259
               ],
               [
                  -0.99468,
                  60.70259
               ],
               [
                  -0.99466,
                  60.7026
               ],
               [
                  -0.99465,
                  60.7026
               ]
            ]
         ]
      }
    }

The third and fourth points are now identical.

A potentially valid solution would simply be to drop any additional points with the same coordinates when forming the final polygons, although this of course would require additional logic.