On Apr 14, 2009, at 4:06 AM, Moritz Lennert wrote:
On 14/04/09 08:37, Vincent Bain wrote:
Hello Adam,
maybe another solution in this case would be a set of 2 tables :
* one linking to the geometry, that is containing nothing but cat
values,
* another one, containing a cat column (related to the "geometric"
table) and different data columns corresponding to your sampling.
I think that if all you want is calculate some means or similar
across dates and then display the results, Vincent's solution is the
easiest.
But you could also use layers [1]:
layer 1 = January round of sampling
layer 2 = February round of sampling
etc.
You would have to give each point a category value in each layer (cf
v.category) and then either create separate tables for each period
linking each to one of the layers or at least create some obvious
cat values (i.e. 100s for January, 200s for February, etc) and link
on single table to all the layers, but with different cat values in
each layer.
Moritz
[1] See "Vector object categories and attribute management" on http://grass.osgeo.org/grass64/manuals/html64_user/vectorintro.html
for a quick introduction
Thanks,
But, the problem with both of these approaches, columns, and layers,
(Vincent or Moritz version) is that I don't have consistent times for
each site. So, at site A I might have 5 samples, once a month and at
site B I have 2 samples, one each year, and site C I have a few spread
over a few years.
So both of those approaches essentially need to have a column, or
layer, for each possible time of sampling. But that is not really
appropriate for the quasi-random times of the samples.
Does this help ?
VB
Le lundi 13 avril 2009 à 14:23 -0700, Adam Dershowitz a écrit :
I am trying to set up a new project in Grass, and I have a
question about the best approach.
I have different vector locations, and at each one there were
multiple samples taken. At the moment I have each sample as a
row in a data base.
My question is how best to put this data into a set of vector
points.
I believe that I can do it in either of two ways (of not others).
1) I can create a vector point at each location, then I think
that I can have multiple cats for that object. So I think I can
do cat=1,3,6 for a given location.
Will that work OK?
2) I can just create different vector objects, that happen to be
at the identical location, and have each one point to a different
cat.
If the above is not clear, here is a bit more detailed example.
At location A there was a sample collected on 1/1 with a value of
2.1, on 2/2 with a value of 2.2 and on 3/3 with a value of 3.3
The above data is already 3 rows in a database.
I want to be able to display data about point A (say, average
value or things like that). Should I just create a vector point
A and then do cat=1,2,3 or should I create 3 different vector
points at A, each one having a different cat?
Any guidance about the benefits or limitations each approach (or
any other approach to consider) would be greatly appreciated.
Thanks,
--Adam
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