How to create a polygon grid using 'irregular' underlying points?

Question

Hello.  I have study plots on the ground, each made up of an alpha-numeric grid.  Since it is impossible to physically lay out a perfect grid in the field, the stakes are not perfectly aligned, and I am using actual grid-stake coordinates in GIS (ArcMap 9.3, but I may have access to 10 if needed).  I want to create a polygon grid over these stake (point) coordinates in GIS, where lines intersect on the actual points.  This is the first step...from here I need to divide each resulting grid square into quarters.  I am a graduate student and something of a novice with Arc, but have tried to teach myself a fair bit over the past year.  A novice-level answer would be much appreciated.  Thanks!

MRpolygons1.jpg

MRpolygons2.jpg

Anonymous User · Answer

Original User: 1812

Since your points are classified/indexed by their positions in rows and columns, as shown in the image, you can try the following (requiring ArcInfo license):

Assuming the fieldname of the point classification is pointClass:
1. Run Add Field tool to add a field, rowID.
2. Run Calculate Field tool to calculate rowID by !pointClass![0]; and check Python in Parser. This should give you an "A" for all A1, A2 ... points, a "B" for all B1, B2 ... points, etc.

3. Run Add Field tool to add another field, say columnID.
4. Run Calculate Field tool to calculate columnID by a !pointClass![1:]; and check Python in Parser. This should give you a "1" for all A1, B1 ... points, a "2" for all A2, B2... points, etc.

5. Run Points To Line tool (new in ArcGIS 10) and specify the rowID as Line Field and pointClass as Sort Field. You should get rows of lines.
6. Run Points To Line tool again and specify the columnID as Line Field and pointClass as Sort Field. You should get columns of lines.

7. Run Feature To Polygon tool (requiring ArcInfo license) using both rowlines and columnlines as input. This gives you the polygons.
8. Run Feature Vertices To Points (requiring ArcInfo license) to get ALL vertices from the polygons.
9. Run Create Thiessen Polygon (requiring ArcInfo license) using the vertices as input. The output polygons go beyond the point area.
10. Run Intersect tool to intersect the Thiessen polygons with the polygons from step 7. Now you have subdivided polygons (quarters).

It's a long workaround. Does it work for you?

SamCoggins1 · Answer

You could tilt the fishnet once it's made. But I understand your grid points aren't regular in any direction. Depending on how the points are named it might be relatively simple to do it with Python. You could also try the points to line tool. This hasn't given me much luck in the past.

Anonymous User · Answer

Original User: larkspurYou can use the Fishnet tool to make a grid that would be the same extent as your grid. Alternatively, if your grid extent isn't large, then just make a new line shapefile (or other feature class) and manually draw the lines that intersect the points. You may be able to draw in the sub-divisional lines after this... ?

Thanks for your answer.  My understanding is that the Fishnet creates a regular grid, which would be off from my slightly irregular grid.  Also, doesn't it run in strictly north/south and east/west directions?  Some of my plots are tilted (i.e. southwest to northeast, northwest to southeast).  I have six plots, each 13 x 13 stakes long.  Maybe I could manually draw the lines that intersect the points (it would take some time), but how then to divide each square into equal quarters?  I'd rather not have to rely on 'eyeballing' it if possible...