How to calculate the area of polygon in WPF Bing map?

Question

Hi everyone, I have a polygon on Bing Map WPF and want to calculate its area in square feet. I used a list contained all points of the polygon to calculate using this formula: area += (point.X * nextPoint.Y - point.Y * nextPoint.X)/2. However, the result
is too large. For example, I calculated a rectangle of 57ft and 80ft and the result was 58000. It's supposed to be 4560. Do I need to convert points to feet? Could you help me to solve this? Thanks a lot.
Here is my code:
double area = 0;
double area2 = 0;
for (int numPoint = 0; numPoint < inputPoint.Count - 1; numPoint++)
System.Windows.Point point = inputPoint[numPoint];
System.Windows.Point nextPoint = inputPoint[numPoint + 1];
area2 += point.X * nextPoint.Y - point.Y * nextPoint.X;
area = area2 / 2;

Answer 1

Calculating the area of a polygon on a map is very complicated as the world is a sphere and you are actually trying to calculate the area of a polygon stretched on the surface of a sphere. Since you are using WPF I'd suggest to make things easy and make
use of the spatial library available in SQL server. All the spatial functionalities in SQL server are available as a dll which you can use in your WPF application. You can easily use this library to calculate the area of a polygon accurately and do a lot of
other really powerful things as well. To start off with, if you have SQL instelled you can find the SQL Spatial Library (Microsoft.SqlServer.Types) located in the C:\Program Files (x86)\Microsoft SQL Server\110\Shared directory. If you
don't have SQL Server installed you can download the express version for free from here:
http://www.microsoft.com/en-us/sqlserver/editions/2012-editions/express.aspx
Take a look at this hands on lab for information using SQL spatial tools in .NET:
http://view.officeapps.live.com/op/view.aspx?src=http%3A%2F%2Fecn.channel9.msdn.com%2Fo9%2Flearn%2FSQL2008R2TrainingKit%2FLabs%2FUsingSpatialDataInManagedCode%2FLab.docx
Once you have this library you can create an SQL Geography object from your polygon. Once this is done you can then use the STArea method to calculate the area of the polygon. There is a ton of other spatial methods available as well which you can use to
create a really powerful mapping application.
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