[GRASS-user] RE: Problem querying layers other than '1' in gi s.m

Moritz Lennert mlennert at club.worldonline.be
Fri Sep 22 10:48:04 EDT 2006

Radim Blazek wrote:
> On 9/22/06, Moritz Lennert <mlennert at club.worldonline.be> wrote:
>> You can actually assign several category values to one object within the
>> same layer. I don't really see the use of that (but I imagine there is 
>> one).
> There are more, examples:
> - Overlapping areas, e.g. a road on a bridge and a road below
> - Many records with different time for the same point

Both of these examples use different layers to store information, not 
different category values in the same layer, or ?

For the first example, I understand the idea of layers (and am not sure 
how you would do this within a database), for the second example, 
couldn't you just have one table with all the records and then do 
queries such as

where cat= and timestamp=


>> It would be interesting to hear how many people actually use layers and
>> how they use them. I personally don't use them at all, as I normally
>> don't mix different types of objects in the same map. Maybe others could
>> give some use cases.
> An example from my last project are targets of bombing (WWII)
> where each target (can be point, line or area) has many attacks
> (dates differ) in one layer (table) and summary table (bombs/target)
> is linked to the second layer.  Here you have both mix of types
> and layers.

If I follow Trevor's argumentation, this should be done by having just 
one category for each vector object (whatever its type) and then a table 
with all the individual attacks. You should then be able to use 
aggregate queries such as

select category, count(attacks) from table_attacks group by category

to get the summary which you should then be able to display.

Trevor, did I understand you right ?


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