Pixel based distances

Question

I need to draw a 50 KM diameter circle on various map scales/zoom levels without having to use any services.

I'm unable to decide what pixel diameter represents 40 KM!!!

So the question is, how do I draw a 40KM diameter circle!???

Please help

GregoryKramida · Answer

Okay, sorry, didn't notice other formulas weren't any good.
Here's a more practical description.

You can use the following pseudocode and develop functions to convert WGS Lat/Lon to WGS Mercator and back:project_wgs_to_mercator(lon,lat){
   x := lon * 111319.490793241; 
   y := ln(tan((90 + lat) * 0.00872664625997167)) * 6378137.0000033; 
}

project_mercator_to_wgs(x,y){
   lon := x / 111319.49079323905;
   lat := 114.591559026167 * atan(exp(y * pi() / 20037508.3428)) - 90;
}

That should be accurate enough to how ESRI does it within micrometers.

The next thing you probably want to do is figure out WGS coordinates of some point within geo-distance of some central point. I recommend to look into Lambert formula, see how to invert it and get the coord based on the bearing. Should be accurate to within meters.

Finally, you want to make a POLYGON-type geometry, with the only ring being a set of points within this distance (i.e. 40 km) of the central point but with a different bearing (remember to convert WGS to WGS Mercator as you're doing it, and then just slap it onto your separate selection GraphicLayer or whatever have you as a Graphic.

Hope that helps.

GregoryKramida · Answer

I'm betting you're using a Mercator projection, so the "40 km circle" won't in fact be a circle. You can approximate with an ellipse that uses the proposed distance function to determine the length of semi-minor and semi-major axes.

RobertScheitlin__GISP · Answer

Andrew,

I don't know if this is any better but here is another way:            private function calculateDistanceFromPixels(tolerance:Number):Number
            {
                var screenPoint:Point = map.toScreen(map.extent.center);
                
                var upperLeftScreenPoint:Point = new Point(screenPoint.x - tolerance, screenPoint.y - tolerance);
                var lowerRightScreenPoint:Point = new Point(screenPoint.x + tolerance, screenPoint.y + tolerance);
                
                var upperLeftMapPoint:MapPoint = map.toMap(upperLeftScreenPoint);
                var lowerRightMapPoint:MapPoint = map.toMap(lowerRightScreenPoint);
                
                var ext:Extent = new Extent(upperLeftMapPoint.x, upperLeftMapPoint.y, lowerRightMapPoint.x, lowerRightMapPoint.y, map.spatialReference);
                return ext.width;
            }