topic Re: Area calculation in planar with WebMercator polygon in Web AppBuilder Custom Widgets Questions

Area calculation in planar with WebMercator polygon

JamesCrandall — Tue, 23 Nov 2021 14:02:14 GMT

JS 3.x

Before I got too far into this, wondering if anyone can point example of calculating more accurate acreage of input geometry of a web mercator aux sphere defined feature service? I am currently determining acreage value calculations with this function that implements esri.geometry.geodesicAreas but we see a discrepancy in values when we use ArcGIS Desktop and apply a planar projection to the same boundary.

getAcreageValueFromPolygon: function (geom) { var geometryService = new GeometryService("https://utility.arcgisonline.com/ArcGIS/rest/services/Geometry/GeometryServer"); var geographicGeometries = []; //if self intersecting, simplify using geometry service if (esri.geometry.polygonSelfIntersecting(geom)) { //if self intersecting, simplify using geometry service geometryService.simplify([geom], function (simplifiedGeometries) { dojo.forEach(simplifiedGeometries, function (simplifiedGeometry, idx) { geographicGeometries.push(esri.geometry.webMercatorToGeographic(simplifiedGeometry)); }); var areas = esri.geometry.geodesicAreas(geographicGeometries, esri.Units.ACRES); acrevalue = new Number(areas[0]); acrevalue = parseFloat(acrevalue.toPrecision(3)); }); } else { geographicGeometries.push(esri.geometry.webMercatorToGeographic(geom)); var areas = esri.geometry.geodesicAreas(geographicGeometries, esri.Units.ACRES); acrevalue = new Number(areas[0]); acrevalue = acrevalue.toPrecision(3); } return acrevalue },

I think I'm simply looking for examples of applying a State Plane projection to the input geometry and calling planarAreas() instead.

Re: Area calculation in planar with WebMercator polygon

JamesCrandall — Tue, 23 Nov 2021 15:52:13 GMT

Init attempt shows odd results.

WKID: 102658

When feature class is imported into an FGDB and projection of wkid referenced above is applied, the area is quite different from when I implement:

var sr = new SpatialReference({ wkid: 102658 }); //102100 102658 102258 3857 2881 https://spatialreference.org/ref/esri/102258/ geom.setSpatialReference(sr); sqFt = geometryEngine.planarArea(geom, "square-feet").toPrecision(7); console.log('sqFt: ' + sqFt)

Areas calculated:

1. ArcGIS Desktop 10.8.1: 61133457.122888

2. planarArea() Javascript&colon; 7273987.555021531

Re: Area calculation in planar with WebMercator polygon

JamesCrandall — Tue, 23 Nov 2021 19:39:13 GMT

Last attempt with cleaned up compare checker. Results in console:

sqFtGeodesic: 61138987.116298884

sqFtPlanar: 7273987.501667344

Code:

geographicGeometries.push(esri.geometry.webMercatorToGeographic(geom)); sqFtGeodesic = esri.geometry.geodesicAreas(geographicGeometries, esri.Units.SQUARE_FEET); console.log('sqFtGeodesic: ' + sqFtGeodesic) var sr = new SpatialReference({ wkid: 102658 }); //102100 102658 102258 3857 2881 https://spatialreference.org/ref/esri/102258/ geom.setSpatialReference(sr); sqFtPlanar = geometryEngine.planarArea(geom, "square-feet"); console.log('sqFtPlanar: ' + sqFtPlanar)

Re: Area calculation in planar with WebMercator polygon

DanPatterson — Tue, 23 Nov 2021 22:01:41 GMT

Just want to be clear.

None of the examples above are for Web-mercator geometry right? they are for the Florida state plane? (projected). I just see a lot of queries about area and length calculations being "off" when using web mercator sourced data

Re: Area calculation in planar with WebMercator polygon

JamesCrandall — Fri, 26 Nov 2021 16:24:34 GMT

All of our map services are sourced as Web Mercator Aux Sphere projection. We're comparing against the same polygon that was projected to StatePlane in Pro and I wanted to get rid of the discrepancy that geodesicArea() produced.

The thing is .geodesicArea() returns very close to what we see when calculating the same polygon in Desktop/Pro that's been projected to HARN NAD83 SP Florida East 901. It's off by a couple of acres:

geodesicArea(): 1400ac

Harn NAD 83: 1403ac

I am simply attempting to reproduce these results in the web application and was hopeful that using .planarArea() would produce the result above.

Re: Area calculation in planar with WebMercator polygon

KarenRobine1 — Mon, 29 Nov 2021 14:28:37 GMT

OK, I think I pulled this off. I did this some years ago, so bare with me.

For Planar setting, you have to use areasandlengthsparameters. Here's my example (sorry, codes not formatted right when I dropped it in..).

//https://developers.arcgis.com/javascript/3/jsapi/areasandlengthsparameters-amd.html
//https://developers.arcgis.com/javascript/3/jsapi/areasandlengthsparameters-amd.html#calculationtype
_getAreasAndLengthsParameters: function(geomArr) {
//https://community.esri.com/message/274372?commentID=274372#comment-274372
//https://community.esri.com/thread/79094
var areaLen = (this._isGeometryLine) ?
new LengthsParameters():
new AreasAndLengthsParameters();
if (this._isGeometryLine) {
areaLen.lengthUnit = GeometryService.UNIT_FOOT; //TODO GeometryService.UNIT_METER=9001 GeometryService.UNIT_FOOT = 9002
areaLen.polylines = geomArr;
} else {
areaLen.lengthUnit = GeometryService.UNIT_METER; //TODO GeometryService.UNIT_METER=9001 GeometryService.UNIT_FOOT = 9002
areaLen.areaUnit = GeometryService.UNIT_ACRES;
areaLen.polygons = geomArr;
}

areaLen.calculationType = this._getAcreageCalculationType(); //preserveShape, geodesic, planar
//console.log("_getAreasAndLengthsParameters() areaLen = ", areaLen);
return areaLen;
},

and how I set it right before I used the geometry service to calculate it....

var areaLen = this._getAreasAndLengthsParameters(resultGeom);

this._geomService.areasAndLengths(areaLen, lang.hitch(this, this._onAreasLengthsFinish), lang.hitch(this, this._onError, "areas"));

Also, fyi: I noticed that the acreages came our slightly different when calculating it via Pro vs Desktop. My calculations matched one and not the other (I can't remember which one)..