What exactly do the parameters of fishnet mean?

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10-31-2020 09:19 AM
New Contributor II

I'm doing some coding in Python with ArcPy with fishnets, and I'm trying to understand more about the tool.

Specifically, the documentation refers to: 

origin_coord

The starting pivot point of the fishnet.

Point
y_axis_coord

The Y-axis coordinate is used to orient the fishnet. The fishnet is rotated by the same angle as defined by the line connecting the origin and the y-axis coordinate.

Point

I'm having trouble understanding, visually, what the origin coordinate is and what the y_axis_coord is. How does this work?

Secondly, I'd like to be able to specify the extent of the fishnet using latitude and longitude, which is an option (I think):

template
(Optional)

Specify the extent of the fishnet. The extent can be entered by specifying the coordinates or using a template dataset.

  • Left—XMin value
  • Right—XMax value
  • Bottom—YMin value
  • Top—YMax value

Extent

How does this option interact with the required parameters of origin_coord and y_axis_coord?

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MVP Regular Contributor

origin_coord is the bottom left corner of your intended output fishnet grid.

y_axis_coord is used to orient the fishnet output.  If you want a 'normal' rectangular fishnet you would just take the same x value of the origin_coord and add 1 unit of measure to the y value of the same origin_coord.

you would set a geographic coordinate system output for the fishnet as an environment setting:

env.outputCoordinateSystem = arcpy.SpatialReference([WKID])

and doing the same with the y_axis_coord as per a planimetric CRS you would have the grid formed of lines of lat long, seen as 'rectangular' in the plate carree/equirectangular proj.

Not used it myself but I think the extent would set the maximum bounding extent of the output if the corner_coord is not specified.

New Contributor II

"y_axis_coord is used to orient the fishnet output.  If you want a 'normal' rectangular fishnet you would just take the same x value of the origin_coord and add 1 unit of measure to the y value of the same origin_coord."


Why does adding one unit of measure make it rectangular?

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MVP Regular Contributor

the orientation of the fishnet is formed from the imaginary straight line drawn between the origin and y axis coord.

In the beautiful picture I've attached, the line is at 0 degrees, meaning no rotation of the fishnet is to be applied.

The second example is a 45 degree angle being formed   between the coordinates, and a resultant 45 degree skew being applied.

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Esri Regular Contributor
MVP Regular Contributor

1000 downloads, nice! Bruce Harold

New Contributor II

Thank you for this extremely helpful (and beautiful!) illustration. I get it now.

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