I have a raster image in GCS WGS 1984. For sampling I want to take a grid of 500 m^2 in my area of interest and pick the centroid of this square. How do I make sure that the grid created with Fishnet tool is 500 m^2 on the ground? I tried with the Cell width parameter but ended up with very large grids. I can't specify the number of rows and columns because the numbers can vary as I proceed with my project.
Yes it is little difficult to create grid in meters using WGS-84
Here is workflow which works fine for me.
- First create a polygon shape file which contains a bounding box of raster.
- Now re-project this this shape file to a projected coordinate system. Selection of projected coordinate system depends on the area where you are working, if you are not sure then just use Cylindrical Equal Area(world) of Arcgis.
- Now use fishnet tool to create grid by using re-projected polygon shape file as "template extent". Don't forget to put "0" for number of columns/ rows and grid cell size is 500.
Now the output file will be in projected coordinates, just re-project it back to WGS-84 and you have your grid.
Hope that will help you.
To be certain that the squares are indeed 500 meters square you need to use a projection that suitably preserves distance and direction everywhere throughout your interest area. To be rather exacting, this is only mathematically possible over a small-ish area of the Earth (definitely not on the whole sphere, to take things to extremes). You don't specify your area of interest in your question, but hopefully it's small enough that you can use a coordinate system that's essentially 2-dimensional, such as a UTM zone. And in ArcMap, as @iRfAn has said, the data must be in this projected coordinate system for the tool to lay out the cells correctly.