How to select 3 points per polygon grid feature from another feature class

AndrewHankinson

I have survey data which is collected on a 1ha grid square, within each grid will be 4 survey points. 

What I am looking to do is select 3 points that fall within a larger grid square, 10ha. 

With how the data has been collected there are roughly 36 points per 10ha grid square. 

The points selected can be random.

I thought about creating random points and some sort of buffer but that is just a no go and also thought about select by attribute but the data doesn't lend itself to getting specifically 3 survey points selected using any one attribute. 

I've dropped a screenshot of how the survey appears with a 10ha polygon grid for reference. 

Any suggestions are welcome. 

 

Screenshot.JPG

VWT2.jpg

jantolihao_1-1737457825801.png

VinceE

Not sure if I understand your exact needs, but you can try something like below. I think a more efficient method would be doing a spatial join in memory and then working with a pandas Data Frame to select three points from each subgroup, but this may get you started.

Code below will randomly select exactly three points within each "box".

 

import sys
import numpy as np

# a list of point IDs to eventually export
point_oids_to_export = []

# Iterate over your sampling boxes
with arcpy.da.SearchCursor('SamplingPointBox', "OID@") as scurs:
    for oid, in scurs:
        # Select the current box. Select all the points with that box.
        box_selection = arcpy.management.SelectLayerByAttribute('SamplingPointBox', 'NEW_SELECTION', f"OID = {oid}")
        point_selection = arcpy.management.SelectLayerByLocation('SamplingPoint', 'INTERSECT', 'SamplingPointBox')
        
        # Get a list of the OIDs of the points within the box.
        oidValueList = [r[0] for r in arcpy.da.SearchCursor(point_selection, ["OID@"])]
        
        # Select three of those OIDs, not selecting the same one twice.
        rng = np.random.default_rng()
        chosen_oids = rng.choice(oidValueList, 3, False, None, 0, False)
        
        # Add those three new points to the running list.
        point_oids_to_export.extend(chosen_oids)

# Select all the collected points based on the long list of OIDs
query = f"OBJECTID IN {tuple(point_oids_to_export)}"
arcpy.management.SelectLayerByAttribute('SamplingPoint', 'NEW_SELECTION', query)

## export your selection from here, or add "arcpy.conversion.ExportFeatures()"