I want to calculate the frequency of observation of an animal in a forest habitat within the different zones of my study area. To do this I will use the simple equation (number of individuals)/(survey effort). I want to do this within a grid of 150mx150m, where a greyscale gradient will indicate the varying population densities.
So far, I have a polygon grid (fishnet) clipped to an area of study of 150mx150m. The survey effort per grid has been calculated using the 'sum line' function. (vector->analysis tools->sum line length)
Picture: a visual representation of the field site (green polygon), with survey trails (pink lines) and observation locations (dark red points).
Next, I want to calculate the number of individuals observed per grid square. The point shapefile representing the observation locations contains a feature in the attributes table called 'cluster size' which is the number of animals that were observed for any observation point. I want to have the total addition of 'cluster size' for all points within each grid cell. Preferably as a feature (column) in the 'sum line' shapefile so that I can then make a simple calculation using the equation (number of individuals)/(survey effort) for population density within each cell of the grid.
I have tried using the point count function found in vector->analysis but this provides only the number of points per grid cell, while I would want the sum of cluster sizes per grid cell. Can anyone suggest how to do this?
I am using QGIS 2.18.7