2

I want to make a 20 mile by 20 mile fishnet grid over the state of Nebraska, with the origin at either the far southwest corner (approx 11,583,008.75W 5,011,971.55N m) or the far NW corner (approx 11,583,491.14W 5,312,014.51N m). I don't think the fishnet tool is working properly the way I am using it, as the gridcells are too small (they are not actually 20 miles square).

I am starting with the Nebraska shapefile downloadable here: https://www.nebraskamap.gov/datasets/nebraska::nebraska-state-boundary/explore?location=41.450388%2C-99.634628%2C7.86

I am adding the NE shapefile to the basemap, and then creating the fishnet with a template extent defined by the NE shapefile, and an x and y cell resolution of 32186.88 meters (20 miles).

Nebraska has an area of 77421 square miles, so there should be approximately 77421/400 grid cells = 193 or so. Figure there's going to be 250 or so, since the fishnet creates a square grid, and Nebraska is not square at the corners. When I do this, I am getting 400 cells, which is way too many, and the cells are too small.

How do I fix this?

If you select the template extent of the above shapefile, the only parameters you need to fill out are cell size width and height, both of which are 32186.88 meters. But,here they are.

My parameters are: Output Feature Class: my_grid.shp

Fishnet Origin Coordinate: X = -11583186.7033 Y = 4865930.3002

Template Extent: west: -11583186.7033 south: 4865930.3002 east: -10609647.7366 north: 5312215.9049

Y-Axis Coordinate: X = -11583186.7033; Y = 4865940.3002

Opposite Corner of Extent; X = -10609647.7366: Y = 5312215.9049

Geometry Type: Poylgon

4
  • 1
    Create Fishnet takes tons of parameters. You'd need to provide what you used to see why the tool did what you asked, and not what you wanted.
    – Vince
    Feb 25, 2023 at 20:47
  • 2
    Have you looked at the results overlaid on NE? Create Fishnet (Data Management) - ArcGIS Pro | Documentation fills the entire extent. NE might be 77,421 square miles, but its bounding extent/envelope covers almost 94,000 square miles. Also, don't use Web Mercator when calculating areas.
    – bixb0012
    Feb 25, 2023 at 22:35
  • Of course I have! I even explained what I saw in my question! Feb 25, 2023 at 22:38
  • 3
    Then focus on the second part of my comment, i.e., don't use Web Mercator. NE having an area of ~77k square miles is based on a geodesic calculation while NE with a Web Mercator projection has an area ~138k square miles due to the distortion of Web Mercator with areas. Using Web Mercator, the envelope of NE is ~168k square miles, which is around 420 fishnet cells of 400 square miles each.
    – bixb0012
    Feb 25, 2023 at 23:42

1 Answer 1

3

By default, the fishnet calculation will be done using the the same projection as the inputs you've provided, in other words Web Mercator, which is inappropriate for really any geoprocessing. In this case, if you run Add Geometry Attributes for Geodesic Area on the outputs of your Fishnet run, you'll see that the actual area of each fishnet cell is approximately 230 sq mi, with big variability between cells.

enter image description here

enter image description here

Whenever working with an ArcGIS tool, I recommend setting many/most environment variables explicitly, or you will often get results that aren't what you want. You can always check the environment variables that are respected by each tool on the documentation page:

https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/create-fishnet.htm

For this tool, the variables include:

enter image description here

The key environments to look at here include the Output Coordinate System and Extent. If you set the Output Coordinate System to an appropriate projection (e.g., the NAD 1983 Nebraska State Plane) and the extent to the extent of the input Nebraska polygon, you will get a fishnet result as you expect, in this case with 264 cells with an area of 400 sq mi each.

enter image description here

enter image description here

Edit:

The above steps work only if the Map Coordinate System has also been set to the desired output projection. The Fishnet parameters and environments must be set after the map's coordinate system had been set:

enter image description here

2
  • 1
    I agree with the approach, but following it with ArcGIS Pro 3.0.3 produces some odd results, like defect results. Specifically, I get one cell near Nebraska and the remainder of the cells in the South Pacific. Is your fishnet overlaying correctly? If so, what are all the parameters on the other window?
    – bixb0012
    Feb 26, 2023 at 16:52
  • 1
    In 3.1.0, it seems that the template extent and output extent will only fill properly if the Coordinate System of the Map has also been set to the desired output projection. I've edited my answer to reflect this.
    – nrp1000
    Feb 26, 2023 at 17:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.