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I have a very good idea of what I would like to do but I really do not know if it is possible with QGIS. I have points that each one corresponds to a group size and I would like to use GIS to sort this data. In fact, in order to conserve independence between my group sizes I would like to keep only one group size per day. Thus, is QGIS capable of analysing all my points and telling me how many duplicates I have within 100 meters radius for the same day? Then, if the software finds more than one point per day, can I run an application that will only keep one group size (randomly). I do not know if I am being very clear but I hope you understand my problem.

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Date    Day Month   Year    Week no Season  Time    EASTING NORTHING    DATUM   Groupsize
14/03/2006  14  March       2006    11  WINTER  09:20   599707  5733317 NAD83   1
02/04/2002  2   April       2002    14  SPRING  14:34   602500  5732500 NAD83   500
05/02/2002  5   February    2002    6   WINTER  09:00   600359  5734295 NAD83   250
06/02/2002  6   February    2002    6   WINTER  08:30   599000  5733600 NAD83   600
07/02/2002  7   February    2002    6   WINTER  18:00   599200  5733400 NAD83   650
11/02/2002  11  February    2002    7   WINTER  14:30   599850  5734150 NAD83   565
12/02/2002  12  February    2002    7   WINTER  17:10   599661  5733382 NAD83   29
13/02/2002  13  February    2002    7   WINTER  09:00   599800  5734200 NAD83   650
16/02/2002  16  February    2002    7   WINTER  10:30   599010  5733850 NAD83   100
16/02/2002  16  February    2002    7   WINTER  10:30   601034  5734740 NAD83   150
16/02/2002  16  February    2002    7   WINTER  10:30   600463  5734418 NAD83   75
16/02/2002  16  February    2002    7   WINTER  10:30   600628  5734783 NAD83   150
16/02/2002  16  February    2002    7   WINTER  10:30   600147  5734358 NAD83   100
20/02/2002  20  February    2002    8   WINTER  12:00   602500  5734200 NAD83   325
26/02/2002  26  February    2002    9   WINTER  18:05   599661  5733382 NAD83   155

Thank you,

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I'm not sure I understand the concept of what you describe as "group size". Could you provide a sketch? If one point is located at 0/0, the next at 0/60, and the third at 0/120, what should happen? –  underdark Apr 6 '13 at 22:30
Each point corresponds to a count of a wildlife species. Unfortunately, I do not understand your other question "0/0", "0/60"... –  Elena Spark Apr 6 '13 at 22:53
Imagine those are the metric coordinates of three points belonging to one group size. –  underdark Apr 6 '13 at 22:59
Well if those three points correspond to counts that were made on the same day then there is a problem with "0/0 and "0/60" and only one count should be kept... –  Elena Spark Apr 6 '13 at 23:02
... and if you decide to keep 0/60, don't you then still have a problem with 0/60 and 0/120? –  underdark Apr 6 '13 at 23:04

1 Answer 1

up vote 4 down vote accepted

Sorry, your introduction is very unclearto me.

You are obviously searching for some "smart" solution, only those python geeks and R-specialists can give you ...

Here my "unsmart" approach:

Assuming you have fields for the species and date. create a new text field with field calculator, concatenate there the species name and the date, so you will have a species/date-id for your points.

Vector -> Geoprocessing tools ->Buffer

build 100m-buffers for all points.

Vector -> Geoprocessing tools -> Dissolve by field

dissolve buffers with your species/date-id. You will now have a multipolygon for each species/date-combination-buffer-cluster. (Dissolve in QGIS is really slow, so better test with a small subset of data)

Vector -> Geometry tools -> Multipart to singlepart

perform this to get single dissolved buffer-clusters for each area.

Vector -> Geometry tools -> Polygon centroid

Create centroids will leave you with one point for each cluster.

This all will of course only work, when your species aren't hopping around in short distances, so you would end up in buffer-chains all across your map.

Not knowing your data and having nothing similar to play around with, this is only a guess and i'm not even sure I understood your question ;)

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Thank you very much. I will try that and see if it answers my problem. –  Elena Spark Apr 7 '13 at 16:01
Dear Bernd V., I only have one species recorded every time but I tried to dissolve with my "Date" field and it did not change anything on my map... Thank you for your time –  Elena Spark Apr 7 '13 at 17:21
now script it, and you have a smart solution! –  nickves Apr 7 '13 at 17:29
@ElenaSpark What do you mean my "did not change anything"? Do you mean that your buffers aren't dissolved? I just made a little test now with some points, and the hole workflow went as i imagined. So i can only guess that the dissolve tool doesn't like your attributes. To be honest, this function sometimes is really crappy. Maybe try to copy your point layer and remove all other fields, try to change the format of your date entries (what do they look alike, by the way, so i can have a try myself) etc etc etc. It HAS to work ;) –  Bernd V. Apr 7 '13 at 18:09
I tried a couple more times and it worked at my last trial... But it gives me polygones and I would like to only have my points sorted. Just the ones from the same date that are selected so that I can sort them... –  Elena Spark Apr 7 '13 at 19:06

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