[GRASS-user] aggregate a field across features and then display related charts and histograms

[David Epstein]

Hello,


I have looked over a lot of documentation but am still having trouble
with this task. I hope somebody on the list can point me in the right
direction!

I have a table of land parcels as polygon features in a postGIS
database. I have another table of regions in the same postGIS database.
There are between 20 and 50 parcels to a region. I can access the data
without a problem (thanks to some helpful people!)

I want to create several maps that aggregate/summarize the parcel data
and display them as pie-charts and histograms at the centroid of each
region. For example:  

1. HISTOGRAM of the size of all parcels in the region. The data for this
is stored in a single field for each parcel. I just need to understand
how to identify the field, send it to the histogram, and display it at
the centroid of the region layer.

2. PIE-CHART where each slice represents the total area owned by a
particular class of owner. This is more complicated. EACH parcel has an
area column for each of 7 classes of owners. So, I believe I need to
keep separate tallies and send them all to a pie-chart function--but I
am not sure how...

Finally, is the best way to actually KEEP all this data to add new
columns to the region table with SQL?


thank you in advance,
-david