[GIS] Random selection within subset

pointqgisrandomselect

I'm working in QGIS 2.18.3.

I have point layer which represents GPS points along a transect line. I would like to randomly select 5 points along this transect, i.e. from this layer.

Somehow I expected this to be straight forward, but I seem to miss something here.

I used Research Tools –> Random selection within subset, selected the transect as input layer, Latitude as ID Field (Latitude = Real), Number of selected features (for the count instead of percentage) and entered 5 as number of selected features.

After pressing "Run" the process runs without problems as the process bar proceeds to 100%, then closes and I receive no error message.

However, nothing has been selected, i.e. no 5 points are highlighted in the attribute table (header still shows selected=0).

Am I using the function in a wrong way or do I expect the wrong output?

Best Answer

One way to do this might be to:

export your subset to another layer
add a field to the attribute table, type float
calculate random numbers using the Field Calculator for each cell
pick the 5 highest (or lowest) numbers.

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[GIS] Georeferencing to x y coordinates with one lat long origin

When you say "the locations of all our logging of other features/events will be recorded relative to the origin" I assume you mean in some planarlinear measurement (lets use meters in this example). The steps you would take, assuming your data is in a spreadsheet, are as follows:

Project your lat/long into a local meter projection (this will depend on your location). Find the easting and northing.
Take your other relative measurements and add/or subtract them to the origin's easting and northing in the spreadsheet.
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ArcPy – How to Select Random Points from Feature Class

Since we need to compile a list of IDs for an SQL IN operator anyway, let's use that string as the comparison basis to generate the unique lists...

import random
import arcpy

numSets   = 100
setSize   = 20
sourceFC  = r'D:\gis_se\pick49.shp'
outputGDB = r'D:\gis_se\picked.gdb'

# Extract all IDs
rawIDs = sorted([row[0] for row in arcpy.da.SearchCursor(sourceFC,'OID@')])

# Compile unique sets
resultSets = []
while (len(resultSets) < numSets):
    candidate = list(rawIDs)
    random.shuffle(candidate)
    subset = ','.join([str(id) for id in sorted(candidate[:setSize])])
    if (subset not in resultSets):
        resultSets.append(subset)

# Export the copies to GDB (or not)
print("\n".join(sorted(resultSets)))

When testing with "generate 10 sets with 5 picked from 10" I was able to generate duplicate candidates, but when I moved up to "generate 100 sets with 20 picked from 49" I wasn't, but even if your random generator is cruel, this won't generate duplicates.

Best Answer

Related Solutions

[GIS] Georeferencing to x y coordinates with one lat long origin

ArcPy – How to Select Random Points from Feature Class

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