calculating total area of a map

[jywarren]

Some notes and starter code for this:

start by merging polygons and discarding all but exterior, largest poly?

dunno how to do that... missing step

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If you want to do this with a more "GIS" flavor, then you need to select an unit-of-measure for your area and find an appropriate projection that preserves area (not all do). Since you are talking about calculating an arbitrary polygon, I would use something like a Lambert Azimuthal Equal Area projection. Set the origin/center of the projection to be the center of your polygon, project the polygon to the new coordinate system, then calculate the area using standard planar techniques.

http://mathworld.wolfram.com/LambertAzimuthalEqual-AreaProjection.html

PROJ4.js: (support was added for Lambert Azimuthal Equal Area projection (proj=laea, ported from PROJ.4))
http://proj4js.org/

-- how to get the result of Lambert proj in meters?

proj_laea=Proj("+proj=laea +lat_0=55n +lon_0=20e +x_0=0.0 +y_0=0.0 +units=m")
proj_latlon=Proj("+proj=latlon")
transform(proj, Proj("+proj=latlon" ), 9999000, 8000000)

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Proj4js.defs["EPSG:27563"]="+title=LAMB sud france +proj=lcc +lat_1=44.1 +lat_0=44.1 +lon_0=0 +k_0=0.999877499
+x_0=600000 +y_0=200000 +a=6378249.2 +b=6356515 +towgs84=-168,-60,320,0,0,0,0 +pm=paris +units=m";

Proj4js.defs["EPSG:????"]="+title=???? +proj=laea +lat_0=55n +lon_0=20e +x_0=0.0 +y_0=0.0 +units=m";
Proj4js.defs["EPSG:8008"]="+title=LAMB azimuthal equal area +proj=laea +lat_0=55n +lon_0=20e +x_0=0.0 +y_0=0.0 +units=m";

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// include the library

<script src="lib/proj4js-combined.js"></script> //adjust the path for your server

// not sure about this, trying to define custom projection at 55 N 20 E... center of polygon, using meters
// Proj4js.defs["EPSG:8008"]="+title=LAMB azimuthal equal area +proj=laea +lat_0=55n +lon_0=20e +x_0=0.0 +y_0=0.0 +units=m";

// creating source and destination Proj4js objects
// once initialized, these may be re-used as often as needed
var source = new Proj4js.Proj(‘EPSG:4236’); //source coordinates will be in Longitude/Latitude
var dest = new Proj4js.Proj(‘EPSG:27563’); //destination coordinates in LCC, south of France

// transforming point coordinates
var p = new Proj4js.Point(-76.0,45.0); //any object will do as long as it has 'x' and 'y' properties
Proj4js.transform(source, dest, p); //do the transformation. x and y are modified in place

//p.x and p.y are now EPSG:27563 easting and northing in meters

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Then use Geometry.poly_area() in Cartagen, result in meters, unsigned probably.

[jywarren]

Rough version: pixel resolution squared times summed pixel area of images

Use poly_area from Cartagen: https://github.com/jywarren/cartagen/blob/master/public/cartagen/src/util/geometry.js

[jywarren]

this would be cool but is not in high demand and would be better done as a Leaflet plugin