topic Re: Draw a polygon given an ordered list of lat/lon points? in ArcGIS API for Flex Questions

Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Wed, 19 Mar 2014 18:54:44 GMT

I have a map, the projection is Web Mercator. My web application is going to be calling (via javascript external interface) a function which will receive an object as input like this:

// JavaScript  var inputObject = {       polygon: [ [40.6701, 73.94001], [40.6724, 73.94012], [40.6902, 74.02011] ],      title: "My Selected Region" }  document.getElementById("myMap").SetSelectedPolygonFunction(inputObject);

...I've already got a graphics layer on the map ("myGraphicsLayer").

Now, what I need to do is write this function:

// Flex map code  // In initialization function: // ....    ExternalInterface.addCallback("SetSelectedPolygon", setSelectedPolygon); // ....etc   private function setSelectedPolygon(data:Object = null):void {    var thePolygon:Polygon = doSomethingOrOtherTo(data.polygon); // What do I do to data.polygon to make this function work?   var myGraphic:Graphic = new Graphic();  myGraphic.geometry = thePolygon;   myGraphicsLayer.add(myGraphic);  }

Re: Draw a polygon given an ordered list of lat/lon points?

RobertScheitlin__GISP — Sat, 11 Dec 2021 07:37:10 GMT

Daniel,

Just writing code of the top of my head, I would try something like this (of course this is missing the imports):

var thePolygon:Polygon = new Polygon(null, new SpatialReference(4326));
thePolygon.addRing(JSONUtil.decode(data.polygon));
var wmPoly:Polygon = WebMercatorUtil.geographicToWebMercator(thePolygon);

Re: Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Sat, 11 Dec 2021 07:37:13 GMT

That seems simple enough -- what exactly is the new SpatialReference(4326) about? Forgive the newbieness.

Daniel,

Just writing code of the top of my head, I would try something like this (of course this is missing the imports):
var thePolygon:Polygon = new Polygon(null, new SpatialReference(4326));
thePolygon.addRing(JSONUtil.decode(data.polygon));
var wmPoly:Polygon = WebMercatorUtil.geographicToWebMercator(thePolygon);

Re: Draw a polygon given an ordered list of lat/lon points?

RobertScheitlin__GISP — Wed, 19 Mar 2014 19:22:42 GMT

Daniel,

Because those coordinates that you are getting are not web mercator, they look like WGS1984 lat Lon coordinates.

Re: Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Thu, 20 Mar 2014 12:46:16 GMT

I figured it was something along those lines. So basically I'm defining the polygon in WGS1984 lat/lon, then using the web mercator utilities to convert it into web mercator coordinates, and then putting it on the graphics layer.

Thanks!

Daniel,

Because those coordinates that you are getting are not web mercator, they look like WGS1984 lat Lon coordinates.

Re: Draw a polygon given an ordered list of lat/lon points?

RobertScheitlin__GISP — Thu, 20 Mar 2014 12:57:24 GMT

Daniel,

Yep, that is right.

Don't forget to click the Mark as answer check on this post and to click the top arrow (promote).
Follow these steps as shown in the below graphic:

Re: Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Sat, 11 Dec 2021 07:37:15 GMT

Getting close. After the addRing call, when I spit out the contents of thePolygon.toString(), I get this:

(com.esri.ags.geometry::Polygon)#0
  hasM = false
  hasZ = false
  rings = (Array)#1
    [0] (Array)#2
      [0] (Array)#3
        [0] 40.6701
        [1] 73.94001
      [1] (Array)#4
        [0] 40.6724
        [1] 73.94012
      [2] (Array)#5
        [0] 40.6902
        [1] 74.02011
  spatialReference = (com.esri.ags::SpatialReference)#6
    vcsWkid = NaN
    wkid = 4326
    wkt = (null)
  type = "esriGeometryPolygon"

... The next line, though (var wmPoly:Polygon = WebMercatorUtil.geographicToWebMercator(thePolygon);) , is breaking. Since geographicToWebMercator returns a Geometry, I need to add an "as Polygon" on the end of it so it knows specifically that this is a Polygon geometry. So I put that in place. But, I'm getting an error on that line, and being as I'm calling this from JavaScript, the error message is unhelpful:

[INDENT]"Error: Error calling method on NPObject.
    at <anonymous>:2:17
    at Object.InjectedScript._evaluateOn (<anonymous>:613:39)
    at Object.InjectedScript._evaluateAndWrap (<anonymous>:573:52)
    at Object.InjectedScript.evaluate (<anonymous>:492:21)"[/INDENT]

..Any idea what the problem might be?

Daniel,

Just writing code of the top of my head, I would try something like this (of course this is missing the imports):
var thePolygon:Polygon = new Polygon(null, new SpatialReference(4326));
thePolygon.addRing(JSONUtil.decode(data.polygon));
var wmPoly:Polygon = WebMercatorUtil.geographicToWebMercator(thePolygon);

Re: Draw a polygon given an ordered list of lat/lon points?

RobertScheitlin__GISP — Thu, 20 Mar 2014 16:12:05 GMT

Daniel,

Well, not much to go one there... So you do have the import for WebMercatorUtil right?

Re: Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Sat, 11 Dec 2021 07:37:18 GMT

I definitely have the import.

So... I stuck it in a try/catch block to figure out what the real error is. Type coercion failed. So, even though thePolygon is in fact a Polygon, the following is throwing a Type Coercion error:

    
var wmPoly:Polygon = WebMercatorUtil.geographicToWebMercator(thePolygon) as Polygon;

...Is this not the right way to convert the polygon to web mercator?

Daniel,

Well, not much to go one there... So you do have the import for WebMercatorUtil right?

Re: Draw a polygon given an ordered list of lat/lon points?

RobertScheitlin__GISP — Thu, 20 Mar 2014 16:36:37 GMT

Daniel,

Sure that is right. For debugging try this to see if you get past this line of code:

var geom:Geometry = WebMercatorUtil.geographicToWebMercator(thePolygon);

Re: Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Thu, 20 Mar 2014 16:46:57 GMT

I get the same error. Is it possible that despite happily printing out its geometry (per above post), thePolygon somehow contains some screwy data that's causing geographicToWebMercator to croak?

Daniel,

Sure that is right. For debugging try this to see if you get past this line of code:
var geom:Geometry = WebMercatorUtil.geographicToWebMercator(thePolygon);

Re: Draw a polygon given an ordered list of lat/lon points?

RobertScheitlin__GISP — Sat, 11 Dec 2021 07:37:21 GMT

Daniel,

That sounds likely. I think you are going to have to parse the JSON further as the more I think about it the addRing is expecting an array of MapPoints not an array of numbers.

once again this is me free handing this code so there maybe mistakes

var mpArr:Array = []; var pArr:Array = JSONUtil.decode(data.polygon); for (var p:int=0; p<pArr.length; p++){     var mp:MapPoint(pArr
[0],pArr
[1], new SpatialReference(4326));     mpArr.push(mp); } thePolygon.addRing(mpArr);

Re: Draw a polygon given an ordered list of lat/lon points?

RobertScheitlin__GISP — Thu, 20 Mar 2014 17:19:02 GMT

Daniel,

The other thing that occurs to me is where are those coordinates suppose to be in the world? For both the lat and Lon to be positive numbers you would be talking about some where in Asia if you are passing Lat then Lon?..

Re: Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Sat, 11 Dec 2021 07:37:24 GMT

That was it! I think there's something up with the values I'm putting in in the first place, but from this point I'm sure I can handle it. Converting each point into a MapPoint makes the conversion system happy.

Thanks!

Daniel,

That sounds likely. I think you are going to have to parse the JSON further as the more I think about it the addRing is expecting an array of MapPoints not an array of numbers.

once again this is me free handing this code so there maybe mistakes
var mpArr:Array = [];
var pArr:Array = JSONUtil.decode(data.polygon);
for (var p:int=0; p<pArr.length; p++){
    var mp:MapPoint(pArr
[0],pArr
[1], new SpatialReference(4326));
    mpArr.push(mp);
}
thePolygon.addRing(mpArr);

Re: Draw a polygon given an ordered list of lat/lon points?

DanielMauer — Thu, 20 Mar 2014 17:57:01 GMT

Oh, those were just random-ish numbers. The actual numbers will be coming out of a database.

Daniel,

The other thing that occurs to me is where are those coordinates shose to be in the world? For both the lat and Lon to be positive numbers you would be talking about some where in Asia if you are paasing Lat then Lon?..