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Re: [udig-devel] Seeking more direction from the uDIG community
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You are right that you can write queries that reference multiple
table at the same time. You have to write a detailed query for one
then do more queries based on that result. If you are looking at a
very complicated query then it is probably easier to make use of the
DB connection.
For snippets on how to make Filters (the real power of a Query) take
a look at:
http://docs.codehaus.org/display/GEOTOOLS/Snippets
and look at the examples under the Filters heading.
FYI. A query currently permits you to restrict the Attributes
retrieved from the table. SO if you are only interested in the
geometry or just the a couple of attributes it allows you to restrict
the features retrieved to only have those attributes. (Save bandwidth).
Jesse
On 10-Jan-07, at 12:58 PM, Murray Richardson wrote:
Thanks Jesse,
Are there some coded examples on working with geotools? I don't
see exactly how I can execute queries on multiple layers using
QUERY and DATASTORE objects, although it's obviously possible from
what you say.
I think getting the database connection and generating custom SQL
would be most flexible, even if it's not very slick. Also, in some
cases I work with massive point files (posted a question about this
a couple days ago) and I think it would be better to have spatial
queries done on the server side. Similarly with time-series data it
is often necessary to add temporary columns, iterate through the
table several times and perform some calculations. Given this I
would happily take you up on some details to go this route, if it
isn't too much trouble for you...
On the other hand, I may find this gets over my head fairly quickly
and just have to implement these ideas without using uDIG for
anything accept for viewing outputs!
Murray
Jesse Eichar wrote:
Right now a layer is a table in postGIS. Using the QUERY object
and a FeatureStore from the layer, it is possible to obtain
features from that table with the attributes you specify in the
query. The types of queries that can be created can be very
complex and can use many spatial operations. However the
datastore is primarily an Access/Query object that allows access
to the features in the PostGIS (or more generally the data source
what ever it may be) for viewing or direct editing. There isn't
an API for adding columns but there is one for creating new tables.
There is not yet an OPERATIONS API yet in Geotools, which is
likely what you are looking for. There are currently 2 ways to do
what you require that I can think of:
1. In order to satisfy your requirements on the server side you
would have to get the Database connection and generate custom SQL
queries and execute them.
2. You can fetch the features to the client using the Datastore
API and its queries, and do the calculations on the client and
write the result back to the PostGIS, also using the Datastore API.
As far as I can tell you can do most of these steps using the
second method. As I said Queries (and Filters) can do some very
complicated work including GeoSpatial and other "normal" queries.
Intersection, differences, etc... can all be done server side
using Filter and Query.
If you want more details on any of these steps let me know,
Jesse
On 10-Jan-07, at 9:32 AM, Murray Richardson wrote:
Hello Community,
Thanks for previous help you've offered. I could use a bit more
direction if possible to determine whether I should carry on with
uDIG for a small project I am working on. I'm finding the uDIG
development learning curve to be a bit steep so far.
I've spent the last few days looking at uDIG (e.g. plugin
tutorial and all the other online resources) and I am trying to
figure out the simplest way to make use of powerful postgreSQL/
postGIS capabilities. Previously I was concerned with
constructing spatial queries on massive point data sets. These
processes are easy if I use a standard postgreSQL interface like
psql or pgAdmin. It seems like it should be a fairly simple task
to construct plugins within uDIG that make use of these and other
RDBMS capabilities through JDBC (if they don't already exist
somewhere). I read an entry on Cory Horner's community page
about extending uDIG functionality to incorporate better RDBMS
capabilities. I am most definitely interested in this because I
am always faced with problems where this type of capability would
be an asset.
To give you an idea of the type of functionality I am interested
in, considering the following two examples:
(1) I am constantly using GIS and RDBMS capabilities separately
(loosely coupled fashion)for information management in the field
of hydrology. For example - I need to summarize the joint
probability distribution of land-use and soil types to determine
an average (lumped) runoff coefficient for nested watersheds
(since runoff is a function of landuse and underlying soil
properties, among other things). I do this by exporting the
polygon intersection of a soils layer, land-use layer and
watershed layer to a RDBMS. Then I use lookup tables to
determine coefficients for individual landuse-soil combinations
and use an aggregate query to determine the mean value for each
watershed. It is nice to then join the mean value back to the
watershed layer and display this in a map. I also use the RDBMS
to do reporting of the individual intersected polygons by
watershed. This type of analysis and reporting is standard in
flood risk assessment and could be very easily streamlined with a
tool like uDIG and postgreSQL.
(2) Storing and accessing spatial-temporal hydrology data is an
emerging area of interest (consider the ESRI ArcHydro framework
if you're familiar with it) and an interesting direction for Open
Source GIS. For example, I would like to be able to store decades
worth of hourly streamflow data for numerous gauged weirs within
a postgreSQL DB and have this linked to a uDIG application that
allows me to extract the streamflow data by (e.g.) clicking on a
point station and entering a date. The query result could be
just exported to a text file for now. This capability would open
the door to a whole range of interesting spatial-temporal
hydrology applications (e.g. recession analysis, climate and
streamflow data interpolation, hydrologic model parameterization).
Based on these two particular examples, could someone perhaps
direct me on how to proceed to develop a very simple proof of
concept for enhanced RDBMS capabilities within a GIS
environment? Consider example 1 for instance - can I create a
uDIG operation that is accessed by right-clicking on 3
highlighted layers (like the difference operation that is
available when two layers are selected). For my purposes I would
like these to be postGIS layers. The operation could then
execute the postGIS intersection procedure, the attribute join
and the aggregate function all within the postgreSQL service, and
then map the results back to the watershed layer for display in
uDIG?
I know this is a fairly specific question, but what it boils down
to is, how do I access postGIS spatial operations as well as
general postgreSQL RDBMS functionality and non-spatial tables
within the uDIG environment?
Thanks in advance for any help. This project is for a small Open
Source/Open Access Student Experience program grant I am working
on through University of Toronto that involves developing a proof-
of-concept idea and some educational tutorials using postgreSQL/
postGIS. I would like to eventually be able to post the results
on my own uDIG community page if things go well. I think this
type of work could help open up uDIG capabilites to the slightly
less technical users like myself (e.g. GIS and basic programming/
RDBMS skills experience, but still need lots of examples and
explicit documentation to follow!).
Look forward to hearing back from someone - hope it's not too
much to ask over a mailing list!
--Murray Richardson, M.Sc
PhD Candidate
University of Toronto
murray.richardson@xxxxxxxxxxx
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