topic Re: Help using NormalizeCentralMeridian (or spanning the antimeridian) in .NET Maps SDK Questions

Help using NormalizeCentralMeridian (or spanning the antimeridian)

MattH — Thu, 18 Aug 2022 21:53:22 GMT

I'm trying to display some geometries that span the antimeridian in a WPF app. Ultimately, the geometries will be pulled from a map service, but I have a simple rectangle polygon that I'm testing with. This seems to be a pretty common issue, and I've read through all of the examples that I could find. Most of the examples I've seen are using the JS API, and I'm having trouble getting the same results in the .Net SDK.

The working JS example here (https://codepen.io/andygup/pen/QeyerP) appears to call geodesicUtils.geodesicDensify() to resolve the issue. I didn't see any reference to NormalizeCentralMeridian() in this example, like I've seen in other examples. Porting that to .Net didn't work, though.

This example converts to WebMercator then uses NormalizeCentralMeridian() (https://jsbin.com/xegicih/2/edit?html,output), but again is in JS.

This post (https://community.esri.com/t5/arcgis-runtime-sdk-for-net-questions/polyline-not-crossing-international-dateline/td-p/496299) is in .Net and suggested adding 360 to the negative western longitudes. Doing this will put a box in the correct place, but with an extra line at 180 degrees. The JS examples don't have this issue.

Can the .Net SDK display a geometry the same way as the JS examples?

Here is a sample of the code I've been working with:

Here's how I tried adding 360 to the longitudes. That got me a bit closer, but still not what I was looking for:

private void addGraphic(Geometry.Geometry geometry) { SimpleLineSymbol outlineSymbol = new SimpleLineSymbol(SimpleLineSymbolStyle.Dash, Color.Green, 2); Geometry.PointCollection points = new Geometry.PointCollection(SpatialReferences.Wgs84) { new MapPoint(-178, -5), new MapPoint(-178, 5), new MapPoint(178, 5), new MapPoint(178, -5) }; Polygon polygon = new Polygon(points); Polygon newPoly; Geometry.PointCollection newPoints = new Geometry.PointCollection(SpatialReferences.Wgs84); foreach (ReadOnlyPart gp in polygon.Parts) { foreach (MapPoint pt in gp.Points) { double x = pt.X; if(x<0){ x += 360; } newPoints.Add(new MapPoint(x, pt.Y)); } } newPoly = new Polygon(newPoints); //this puts a box in the correct place, //but has an extra vertical line at 180 esriMap.SetViewpointGeometryAsync(newPoly.Extent, 50); Graphic graphic = new Graphic(newPoly, outlineSymbol); _overlay.Graphics.Add(graphic); }

Re: Help using NormalizeCentralMeridian (or spanning the antimeridian)

MarcHillman — Mon, 13 May 2024 04:46:33 GMT

I have EXACTLY the same problem (although I'm coding in VB.NET). Did you ever get an answer? When I apply NormalizeCentralMeridian to my polygon, nothing happens. I don't get two polygons returned.

Re: Help using NormalizeCentralMeridian (or spanning the antimeridian)

MattH — Mon, 13 May 2024 12:03:25 GMT

Would you believe that I can't remember? I usually post updates if I find answers, so it's possible that I just had to move on.

I'll dig through my old project this afternoon, and see if I can find it.

Re: Help using NormalizeCentralMeridian (or spanning the antimeridian)

MarcHillman — Mon, 13 May 2024 12:15:40 GMT

Thanks - really appreciate that. I'm having 2 difficult problems, and NormalizeCentralMeridian seems to be doing nothing.

1. I have a simple rectangular box spanning the antimeridian. Can't split it in two with NormalizeCentralMeridian.

2. And then I have the mother of all meridian problems. I have a polygon enclosing Antarctica. Need to split that in two as well.

Re: Help using NormalizeCentralMeridian (or spanning the antimeridian)

MarcHillman — Thu, 16 May 2024 05:46:15 GMT

This problem is REALLY bugging me. I can't see what I'm doing wrong. I have written a minimal code example. It creates a linestring (using a polylinebuilder) that crosses the Antimeridian (it's a box around Fiji). It is 1 part with 5 points. I then apply NormalizeCentralMeridian to it to produce "normal". normal is identical to linestring.

Dim plb As New PolylineBuilder(SpatialReferences.Wgs84) ' drawn bounding box plb.AddPoint(New MapPoint(177.17, -19.25)) ' first point plb.AddPoint(New MapPoint(-179.65, -19.25)) plb.AddPoint(New MapPoint(-179.65, -15.67)) plb.AddPoint(New MapPoint(177.17, -15.67)) plb.AddPoint(New MapPoint(177.17, -19.25)) ' last point Dim linestring = plb.ToGeometry AppendText(TextBox1, $"Before {linestring.ToString}{vbCrLf}") Dim normal = linestring.NormalizeCentralMeridian AppendText(TextBox1, $"After {normal.ToString}{vbCrLf}")

Output of test program shows NormalizeCentralMeridian is having no effect

Before Polyline[Parts=1, Points=5, Wkid=4326] After Polyline[Parts=1, Points=5, Wkid=4326]

Re: Help using NormalizeCentralMeridian (or spanning the antimeridian)

MarcHillman — Thu, 16 May 2024 14:48:18 GMT

I have a fix, but I'm not why it's valid. If I have a polygon that spans the antimeridian, I simply add 360 to any -ve X value. NormalizeCentralMeridian then does what you'd expect. I'm not sure I fully understand why, but at least I have a solution.

Re: Help using NormalizeCentralMeridian (or spanning the antimeridian)

MattH — Thu, 16 May 2024 16:09:30 GMT

Do you have a snippet of code that you could post? That sounds like what I was trying, but I'm still getting an additional line along the antimeridian.

Re: Help using NormalizeCentralMeridian (or spanning the antimeridian)

MarcHillman — Fri, 17 May 2024 13:39:22 GMT

Below is the code I currently use. It works in almost all cases.

1. If you have a polygon spanning the antimeridian, it produces the desired result - the polygon is split into pieces. I have a box around Fiji, and I can render it no problem.

2. I also have a polygon that encircles the South Pole. This code is splitting the polygon into 3 pieces, but it is not rendering properly. All the signs on the X's in the 3 pieces are the same, so they look sensible.

The code works by testing for antimeridian crossing (CrossesAntiMeridian) and if it does then

1. deconstruct the polygon into a polygonbuilder so we can modify it

2. Add 360 to every X that is less than 0 in every part

3. Rebuild the polygon from the polygonbuilder

4. Use NormalizeCentralMeridian

5. Use Simplify as results of NormalizeCentralMeridian seem to leave polygons that are not Simple

As I said, it seems to be working for 'standard' cases where a pole is not involved, but it seems to breakdown at the poles.

Function CrossesAntiMeridian(g As Geometry) As Boolean ' returns true if geometry crosses anti-meridian Return Math.Abs(g.Extent.XMax - g.Extent.XMin) > 300 End Function Function NormalizeCentralMeridian(PolyBox As Polygon) As Polygon ' Normalize a polygon if it crosses the anti-meridian If CrossesAntiMeridian(PolyBox) Then ' crosses anti-meridian With PolyBox.Extent Dim plb As New PolygonBuilder(PolyBox) ' deconstruct polygon to polygon builder ' Add 360 to any negative X value in any part For prt = 0 To plb.Parts.Count - 1 For pnt = 0 To plb.Parts(prt).Points.Count - 1 If plb.Parts(prt).Points(pnt).X < 0 Then plb.Parts(prt).SetPoint(pnt, New MapPoint(plb.Parts(prt).Points(pnt).X + 360, plb.Parts(prt).Points(pnt).Y, plb.SpatialReference)) ' add 360 to X End If Next Next Dim poly = plb.ToGeometry ' reconstruct geometry Dim Normalized As Polygon = poly.NormalizeCentralMeridian ' split geometry into parts that don't cross the antimeridian Normalized = Normalized.Simplify ' Normalizing seems to produce a non simple result Return Normalized End With Else Return PolyBox ' doesn't cross anti-meridian End If End Function

Below