[GRASS-user] Multiple to one question

[Adam Dershowitz, Ph.D., P.E.]

On Apr 16, 2009, at 3:02 AM, Moritz Lennert wrote:

> On 16/04/09 06:29, Adam Dershowitz, Ph.D., P.E. wrote:
>> I really liked the suggestion, so I tried it....but it doesn't  
>> work. To continue my simple example, I did  create view less2  
>> cat,Value
>> from data where Value < 2; So I end up with a view, as expected. The
>> problem is that if I now link my map to that view, I get an error: No
>> attribute found for cat 1 (since the object still tries to link to  
>> 1).  And, the same for 2 and 4.  And, I still see the points and
>> slashes (ie it still draws point a and puts a null value then a slash
>> then 1.1. So is there a way to have it not draw a symbol and not draw
>> "null text" when it can't find an attribute?  Since, the whole point
>> of what I am trying to do in this case is to eliminate certain data
>> with a query?
>
> What do you mean by "eliminate" ?
>
>
> Maybe you should be more precise about your objectives. Are you  
> trying to display something on a map, or not ? If all your looking  
> for is tables, than you don't need GRASS at all.

Hmmm, I guess I was not that clear.  Sorry, and thanks for your help  
and patients.

What I mean is that I have a map that has a bunch of sites on it, and  
values shown next to each site as a label.  In many cases there are  
multiple values shown.

What I want to do is draw a second map, where I only show sites where  
the value is greater then a certain threshold.  In other words, my  
first map has all values shown.  My second map should just show sites  
where value is, for example, greater than 2.0 (or whatever).

So what I ended up doing was copying my original map, and then linking  
my new map to a view of the original data.  But that didn't work  
because then all the sites that are below the threshold (and therefor  
are not in the view) don't have a correct value or cat, but the icons  
are still shown, and when there were multiple values in the original a  
slash is still drawn, since it seems that it is using a "null" value  
for the labels.


>
>
>
> On 16/04/09 07:35, Adam Dershowitz, Ph.D., P.E. wrote:
> > It seems like I should be able to edit the cats, and I tried a few
>> different things with v.edit, but I have not had any luck.  I tried
>> some things like this: v.edit map=foo tool=catdel -r where="cat >  
>> 0" but it gives me: ERROR: Tool catdel requires option cats  
>> Although the
>> above seems to work with tool=select, which I was using to test
>> things about before doing the delete.
>
> As the message says, tool catdel does not work with a select, but  
> you have to give it a list of cats. You can do that with something  
> like this:
>
> v.edit map=foo tool=catdel cats=`v.db.select -c foo col=cat  
> where=XYZ | awk '{printf"%i,",$1} END{print}'`
> (note the back ticks)

I ended up doing something pretty similar to this last night, although  
I ended up using environmental variables to store the select results.   
And it accomplished half of what I need.  The problem is that all  
sites are still shown, even if they no no longer have any values, in  
the cases where the edit has eliminated all of them.  So if a vector  
point had a value of 1.0, it is still drawn.

If I follow the above with:
v.edit map=foo tool=delete -r where="cat > 0" then it also eliminates  
the sites that no longer have any categories.  So I guess I have  
things working, but it feels like a kluge, and there should be a  
cleaner solution.

>
>
>> I copied my original vectors, and created the view.  Now what I want
>> to do is to delete all the categories in this vector that point to
>> null. Is there a way to do that?
>
> Why do you want to do that ? If I understood you correctly, all of  
> these cats have a meaning. Maybe not in the currently linked view,  
> but in the original table. So if you delete these cats, you will  
> lose the link to the values in the original table.
>
> Again, I think we need a better explanation of what it is you are  
> trying to get at.
>
> Moritz


Here is a bigger picture explanation.

A bunch of samples were taken at different locations and different  
times.  I want to be able to generate a few different maps:
1)  Show all sampling locations (that one is easy and already done).
2)  Show all sample locations, and values, where value is > 2.0, for  
example.  In other words, if the value at a location is below 2.0 then  
don't display the value, and if all the values at that location are  
below 2.0 don't even display the icon for the location.
Essentially what I am trying to do is to put a red dot where ever the  
I had a measurement above 2.0, and also list those values by the red  
dot.

So far I have not been able to get the above using views and linking  
to those views, because GRASS still "knows" about the points that are  
below 2.0 and tries to display them even if they are not in the view  
and treats them as null values, rather then as values to be ignored.

So far the solution that seems to work is:
1)  Create my master map showing all locations and values.
2)  Make a copy map that I will then edit.
3)  In my copy map use what you had above, v.edit tool=catdel etc., to  
delete the categories that don't meet a query.
4)  Delete any points from my copy that now don't have any cat  
remaining using v.edit map=foo tool=delete -r where="cat > 0"

So, I think that the above does accomplish what I need.  But it was  
pretty round-about to get it figured out and working.  I figured that  
there would be an easier way to do a query and just show sites and  
values that are returned by the query.

--Adam