# Find area of polygon on a sphere from a set of latitude-longitude points

Given a list of latitude and longitudes say

{{32.6123, -117.041}, {40.6973, -111.9}, {34.0276, -118.046},
{40.8231, -111.986}, {34.0446, -117.94}, {33.7389, -118.024}}


how can I find the area enclosed by the polygon formed by these points. In other words, how can I implement the 3D convex hull (surface area) on a set of lat-long pairs?

-

## 1 Answer

To find the area of these points, we first must convert to a projected coordinate system. Since your points lie in multiple state plane systems, we'll use UTM zone 11 to convert your latitude longitudes.

Here are the points courtesy of new Wolfram Language functions...

latlons = {{32.6123, -117.041}, {40.6973, -111.9}, {34.0276, -118.046},
{40.8231, -111.986}, {34.0446, -117.94}, {33.7389, -118.024}};

GeoGraphics[GeoMarker/@latlons]


To do the conversion:

utm = GeoGridPosition[GeoPosition[#], "UTMZone11"][[1]] & /@ latlons


{{-3848.25, 3.60975*10^6}, {431132., 4.51949*10^6}, {-96605.2, 3.76722*10^6}, {423059., 4.53305*10^6}, {-86797.8, 3.76901*10^6}, {-94892.4, 3.73517*10^6}}

This returns coordinates in meters.

Now, using this code:

GraphicsMeshMeshInit[];
PolygonArea[utm]


7.43416*10^10

And let's convert to acres, lazily...

UnitConvert[Quantity[%, "meters squared"], "acres"]


1.83701*10^7 acres

Even so, these points are spread far apart in a strange order across multiple defined projected areas, so the area calculation may not be as accurate as possible.

-
Thanks. Is there a webpage that explains the state plane systems referred here? If I understand right, in the UTM zone 11, all points can be assumed to lie in a plane with small error, and the coordinates -3848.25, 3.60975*10^6 etc are w.r.t. to the boundaries of the zone. Is this correct? – HerbertSBrown Jul 1 '14 at 0:05
@HerbertSBrown Try this: geology.isu.edu/geostac/Field_Exercise/topomaps/utm.htm – kale Jul 1 '14 at 3:26