Hi all,

I am trying to find out circular and rectangular geofence logic in C.
(For circle, I will have fixed radius and centre coordinate)

Please help me with some links/suggestions/replies.

Thanks,
Tapas

What do you need to do, compute perimeter and area? Render the area on a map? Or determine coordinates which define a geometric shape?

If you have the coordinates for say a circle's center or the vertex's of a polygon, you're best first step is to convert those coordinates to decimal degrees and fractions of degrees. From there determine the conversion from the significant value of a GPS digit into distance. Then it's geometry calculations.

Example: Circle

You have center point. You know the radius. Due North, South, East, or West are simple additions or subtractions to either Latitude or Longitude starting from that center point. The 45's NE, SE, NW, SW can be found by using the sine or cosine functions. In the case of the 45, they're equal. To get different angles around the circle, follow the same method. From there you know how to calculate the coordinates by adding/subtracting to both lat and long. In fact for the circle, you only have to do 1/8th of it. Why? You know the zero degree deflection, you calculate the 45 degree deflection. Then if you want say every 5 degrees, you use sine and cosine to determine the X-Y offsets from your center point. Then to get the 45-90 chunk you swap the X-Y values you already calculated. For the other quadrants, you swap the signs. So by calculating 7 triangles, you can compute coordinates for a circle to the precision of 5 degree increments.

1 members found this post helpful.

Thanks ...

So, for a static circlular fence I can check/compute the coordinates with precision of 5 degrees (lets say). Then I can capture those points for evalutating individually with the current location of the subject.

So it is virtually a circle but can be told as a polygon sets of points, right?

Yes, and really all circles can be considered a polygon of multiple points.