Distance between points with transform about projnet4geoapi HOT 12 CLOSED

I can't show the original two points but I chose two arbitrary points roughly ten km from each other. I'm getting roughly the same result.

point a: POINT (-77.220888 39.168931)
point b: POINT (-77.150249 39.219669)

degreesDistance: 0.086972483953261043 (this I get just using the .Distance() method on the IPoint)
degreesToMeters: 9682.2117760967849
transform: 10701.365215911384 (wgs84 to Popular Visualisation CRS / Mercator)
haversine: 961.83508919368637 (i suspect there's something wrong with the formula here)
PostGIS: 8376.15057906345
meridianoutpost.com: 8.3 km
.movable-type.co.uk/scripts/latlong.html: 8.3km (haversine calculator)

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You can calculate the UTM zone using the following formula:

public static int CalcUtmZone(double lon) => (int) ((lon + 180.0)/6.0 + 1.0);

You need to make sure that the used projection is suitable for the input points.

SharpMap has a geospatial math utility class that might help

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Could you post the actual coordinates of pointA and pointB?

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I'm afraid Lambert_Azimuthal_Equal_Area projection is not supported (in targetWkt).
How did you get a result in the first place, I get NotSupportedException?

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Oh I did. It's just one of the many projections I tried. I was going through them one by one copy/pasting the WKT of any that had a unit of 'meters' trying to get something that made sense.

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var originCoordinateSystem = GeographicCoordinateSystem.WGS84;
var targetCoordinateSystem = ProjectedCoordinateSystem.WGS84_UTM(18, true);

var transform = transformationFactory.CreateFromCoordinateSystems(
    originCoordinateSystem, targetCoordinateSystem);

var pointA = new GeoAPI.Geometries.Coordinate(-77.220888, 39.168931);
var pointB = new GeoAPI.Geometries.Coordinate(-77.150249, 39.219669);

var pointACoordinate = new GeoAPI.Geometries.Coordinate(pointA.X, pointA.Y);
var pointBCoordinate = new GeoAPI.Geometries.Coordinate(pointB.X, pointB.Y);

var newPointA = transform.MathTransform.Transform(pointACoordinate);
var newPointB = transform.MathTransform.Transform(pointBCoordinate);

double result = newPointB.Distance(newPointA);
Console.WriteLine($"Distance between {pointA} and {pointB} = {result}");

produces

Distance between (-77.220888, 39.168931, NaN) and (-77.150249, 39.219669, NaN) = 8305,08674556441

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Hrm....Getting a 'No support for transforming between the two specified coordinate systems' exception on the line where we call .CreateFromCoordinateSystems().

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Also hrm... looking at what Universal Transverse Mercator coordinate system for zone 18.... (first time I've heard of it but google provides)

How do you determine what zone the points you're looking at are in? What if one point is in one zone and the second point is in another zone?

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I have a similar class that calculates distance using similar methods. I'm just trying to understand how to do it using transforms. I had guessed that you transform both points into a projection that has it's units in meters and then just get the distance between them. Is that not how it works?

I wanna say I appreciate your help. I hate not understanding things.

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To compute the distance between two projected coordinates, you need to make sure that

• both are projected into the same spatial coordinate system
• the used spatial coordinate system is suitable for the distance calculations (it does not make sense to use EPSG:31467 when your coordinates are in Australia)

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right oranges to oranges.... but they are, aren't they? They're converted to WGS84 UMT... but then there's zone. Do you just put them in the same zone? or does that not matter because it's all WGS84 UMT?

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A different UTM Zone is a different spatial reference system. The error might be negligable when you have neighboring UTM zones, but as a rule of thumb, avoid that.

See https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system#Overlapping_grids

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