Random point generation

Question

Hi all,

Please excuse this question from a GIS novice. I have loaded 3 shapefiles into my GIS; one is a polygon file representing various forest classes (A), the other (B) is a point file with 119 points,and the 3rd (C) is a single point location. For each of the 119 points in B I would like to generate 1000 random points that are the same distance from C. Then I would like to know the forest class polygon in A that each of the resulting 119,000 points falls within.

If someone could suggest a solution I'd be most grateful. Thanks.

DavidBradley · Answer

Thanks for your suggestion, Bill. I was looking to generate large number of random points for each bird location, each the same distance (D) as the bird from the release site, to assess whether the birds were located in a particular habitat more often than would be expected by chance. I could do this the painfully long way by randomly choosing angles and then using Pythagorean theorem to determine the locations of points with the same hypotenuse length as D. This is an very inelegant solution, so that's why I posted my question to the Spatial Analyst forum. If there was a simple way to do this somewhat like the method to generate random points within a polygon but with a condition that each point must be D from point A, this could work.

I think your solution would work, but as the grid cells may overlap multiple habitat classes within the habitat class layer, I'm not sure what the output would look like. Assessing habitat class at specific points seems more practical. Doing so at 1000 points would surely be a good approximation of the true proportions of 'available' habitat classes.

DavidBradley · Answer

Do you really need random points, Dave?  Or are you proposing this as a way to estimate the proportion of each class as a function of distance from point C?

Hi Bill. You're right, I basically want to know the proportion of each forest class at the same distance from point C, for each of the 119 points in B. The 119 points are locations of bird sightings and point C is a release location, and I want to know whether the habitat they were in was proportional to what was available, given the distance the birds were from the release site.

Is there an easier way of doing this that creating random points?

Thanks for any help you can give here.
Dave