Finding shortest distance between two polygons(SqlGeography c# )

Question

I want to find the shortest distance between two SqlGeography polygon. I know there is a method ShortestLineTo (https://msdn.microsoft.com/en-us/library/ff929252.aspx) but it gives empty string while doing so. Can anyone suggest me some alternate way to do so?

Elenoraelenore

Can you post your code sample? – Ambassador

added sample. Please look at the image. – Elenoraelenore

provide you an answer. – Ambassador

Answer 1

You should define a GEOGRAPHY in Counter Clockwise order. If you define it CW, it will be all the world except the region you are defining:

DECLARE @G1 GEOGRAPHY = 'POLYGON ((1 1, 3 1, 3 3, 1 3, 1 1))';
DECLARE @G2 GEOGRAPHY = 'POLYGON ((45 45, 45 46, 44 46, 44 45, 45 45))';

//This is what you have defined
DECLARE @G3 GEOGRAPHY = 'POLYGON ((45 45, 44 45, 44 46, 45 46, 45 45))';

@G1: point order is CCW so @G1 is a polygon containing POINT(2,2)

@G2: point order is CCW so @G2 is a polygon containing POINT(44.5,45.5)

@G3: point order is CW so @G3 is a polygon containing the entire world except the @G2 polygon. It also contains the @G1 polygon, so the shortest distance between @G1 and @G3 doesn't have any meaning.

Toggle Between CW and CCW geometries

Using the ReorientObject() method, you can toggle between CW and CCW geographies. So if you try:

SELECT @G3.ReorientObject().STDifference(@G2).STAsText();

The result would be GEOMETRYCOLLECTION EMPTY because they contain the same region. So they intersect and the result of ShortestLineTo returns a LINESTRING EMPTY because they intersect each other.

One more point

You can check if a polygon contains too large of a region by checking the EnvelopeAngle so you are made aware of mistakenly defined geographies.

As shown in the picture, EnvelopeAngle=180 means that polygon contains a very large region of the world.

Ambassador

@Elenoraelenore glad to be helpful :) – Ambassador

Answer 2

Use ShortestLineTo

An empty LineString instance is returned when the two geography instances intersect each other.

Irv

Problem is that two geography instances are not intersecting. See the image to get a clarity. – Elenoraelenore

@Elenoraelenore they do intersect, So LeMara is pointing to the correct issue but for more explanation you may look at my answer. – Ambassador