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Create multiple polygons of equal area from existing polygon

HannahFaulkner1

I have a single polygon that follows a large stretch of shoreline, extending 500 m into the water (with a shape length of 3404910 ft and area of 2559460882 ft). From this single polygon, I want to create multiple polygons of equal area (1000 km2). I need to maintain the intricate boundaries of the polygon, as opposed to creating a grid. It has been suggested to use parcel editing, but I'm afraid the process to convert my polygon to topology to parcel fabric and back out to (something) for further geoprocessing might be beyond what I need. Is there any easier way to achieve this?

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MarkBoucher

Another approach.

Treat your polygon like a watershed.
Create a raster using your polygon. It needs to have a z component.
Make "centerlines" (see pdf). I recently ran across a forum post about creating centerlines from bounding lines. (here�??s' one, I think there is a more recent one also: http://forums.arcgis.com/threads/76532-Creating-Automatic-polyline?highlight=centerlines)
Make "cross sections" in the same polyline layer. There should be an automated way to do this.
Use the new polylines as the agreestream layer for hydrology analysis in Arc Hydro.
Clip the agreestream with your polygon. You may need to make a smaller polygon (use offset or buffer) to clip the agree streams layer so the lines don't cross the polygon boundary.
Add one or more polylines that cross your original polygon boundary at the "outlet(s)"
From here apply the Arc Hydro work flow (you need Spatial Analyst (SA) and could use SA tools instead):

Recondition the dem using the agreestream layers to burn them into the raster.
Build walls using your original polygon as the outer wall. Use the agreestream layer to breach the wall.
Fill the sinks. Not sure if you need to do this since the raster is "flat".
Run the flow direction tool.
Run flow accumulation tool.

You will now have a flow direction grid that you can use the hydrology functions in to delineate watersheds. The flow accumulation layer can be looked at to identify locations where the accumulation is a multiple of 1000 km2. Maybe then you can delineate the raster at these locations.

You will have rough edges on the raster, but if the raster is fine enough these should be minimal. I would use model builder as you iterate to refine the steps of this process.

Hope this gets you closer to your goal and good luck!

Concept Sketch.pdf

Label_Halo_Shift_10.1_no2.JPG

MarkBoucher

I believe this would only work if I had a smoother polygon.

Can you simplify your polygon so there are not so many irregularities and then work with a template?

HannahFaulkner1

MBoucher21,

I am hoping to maintain an equal area, as much as possible to eliminate sources of error in my following analysis. These multiple polygons will then act as a study area grid to overlay points, then export and analyze tabular data.

I think I'm going to have to get create somehow, or as you mentioned, eliminate the strict need of producing polygons of equal area. What then would be the best method if I were to fudge on the 1000 km2 area requirement.

HannahFaulkner1

So I can best understanding what you're advising, you are suggesting:

that to separate my polygon into multiple adjoining polygons to create a template polygon of my desired area (1000 km2) then head down the shoreline clipping each bit by my template polygon. I believe this would only work if I had a smoother polygon. Clipping one stretch of shoreline wont produce the same area as say clipping a tiny inlet.

I'm having real trouble conceptualizing what to do here. It seems that in parcel editor you can simply divide parcels by a certain desired area. But to transfer my polygon to a parcel fabric seems like it may create more hassle.

MarkBoucher

I want to create multiple polygons of equal area (1000 km2).

Hannah,

I would ask the person who wants this (even if it that is you), "Is there something special about the 1000 km2? Can the areas be approximately 1000 km2?"

Seriously, for you to accomplish this task with the spaghetti of a polygon you sent would seem to be impossible or at least expensive (time and money) to do it manually. Determine if there can be some leeway in this target of 1000 km2.

RobertBorchert

What I mean by copy and paste is you create the FIRST polygon in the size and area that you want. Then you copy it and paste it and move the pasted polygon so it edge matches the first polygon.

However, on seeing your shoreline it would be quite messy.

Possibly you could now create a new polygon of the desired size in a new feature. Then use that new polygon to clip the original polygon. You would have to rotate is and snap it to the edges of the remaining uncliped polygon.

I cannot have overlapping polygons, if that's what you mean. I need to chop up a single snake of a polygon to separate adjoining polygons of equal area. Yes, this means that the polygons will touch but jiggle up and down the shoreline. I am hoping to choose this method instead of creating polygon at every certain distance along the shoreline to account for bays and inlets.

I don't understand what you are saying about the copy and paste. I believe that would just replicate the issue, because I only have one polygon that I am trying to make say 200 polygons out of.

The polygon was created originally from the shoreline, but applying a 500 m buffer, then splitting that buffer polygon by the original shoreline and excluding upland 500 m strip and islands.

Attached is a PDF of the polygon I'm working with so you can get a better understanding.

Thank you so much for your help!

HannahFaulkner1

I cannot have overlapping polygons, if that's what you mean. I need to chop up a single snake of a polygon to separate adjoining polygons of equal area. Yes, this means that the polygons will touch but jiggle up and down the shoreline. I am hoping to choose this method instead of creating polygon at every certain distance along the shoreline to account for bays and inlets.

I don't understand what you are saying about the copy and paste. I believe that would just replicate the issue, because I only have one polygon that I am trying to make say 200 polygons out of.

The polygon was created originally from the shoreline, but applying a 500 m buffer, then splitting that buffer polygon by the original shoreline and excluding upland 500 m strip and islands.

Attached is a PDF of the polygon I'm working with so you can get a better understanding.

Thank you so much for your help!

shoreline_polygon.pdf

arcpymapping.py.zip

RobertBorchert

Do your polygons need to go in a straight line?

Are they following the shore line? Meaning the polygons need to touch be their position may jiggle up and down as it follows the shoreline?

Either way just simply use the copy and paste functions.

It will reproduce your polygon perfectly. Great for making map pages.

Use snapping. if they need to be perfectly lined up simply snap to an edge or a vertex.

How was the original polygon generated? Perhaps it would be more simple to redevelop the polygon but in a different manner so it is equal area.

Perhaps you could post a screen capture of what your polygon looks like.

I have a single polygon that follows a large stretch of shoreline, extending 500 m into the water (with a shape length of 3404910 ft and area of 2559460882 ft). From this single polygon, I want to create multiple polygons of equal area (1000 km2). I need to maintain the intricate boundaries of the polygon, as opposed to creating a grid. It has been suggested to use parcel editing, but I'm afraid the process to convert my polygon to topology to parcel fabric and back out to (something) for further geoprocessing might be beyond what I need. Is there any easier way to achieve this?

Changing Zone Color on parcel.png

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