[Gwt-openlayers-users] Fwd: WFS Operations

[Edwin C. <com...@gm...>]

Hi Cameron,

First, a correction. I checked and the WFS class  was already in OpenLayers
2.7 and OL versions before that. Although it is true that there were many
enhancements relating to WFS in OpenLayers 2.8.

Anyway, I remember why it is relevant to know about the proxy. For instance,
I work on Windows, and the proxy is a python script that is set up to run
under Linux. The problem is: when the proxy doesn't work, the WFS stuff
doesn't work.

You need a proxy to do WFS requests, because of cross-domain restrictions on
Ajax requests. There are solutions for doing cross-domain GET requests, but
doing cross-domain POST requests can only work with a proxy as far as I
know.

Furthermore, the proxy that comes with OpenLayers is not very sophisticated
and may even present a security risk when it is accessible outside your app.
See also the comments in the proxy.cgi file.

For GWT-OL it makes sense to have a proxy servlet in java. I may need to
look into that. Preferably, I want to keep GWT-OL client-side. Maybe, it is
necessary to start a "GWT-OL server" project that provides server side
components, such as a WFS proxy and a print servlet.

Due to the cross-browser restrictions issue I had not looked deeply into the
WFS.

Now onto what you describe. If you want to shade the map based on what
software version clients are using then you need SLD's. You probably want to
proxy the WMS call that OpenLayers does, and let the proxy do a WMS call
with an SLD that shades the map based on the version of the software. This
is not trivial unfortunetaly. We are using GeoTools for the SLD building in
a custom proxy that is proprietary.

Why would you want to manually loop through polygons on the map? If for
instance the polygons are the states of the USA and they all have a property
CAPITAL, then you would want to store the polygons in a spatially enabled
database (PostGis or Oracle Spatial are the most common ones). A WMS server
could draw a map from those polygons. When someone clicks the map you either
do a getFeatureInfo request on the WMS to get the property, or you pass the
coordinate to the database and let a spatial query sort out in which polygon
the coordinate falls.

I am not reading any use cases for WFS in your mail. WFS is also a bit
tricky, because you can kill the browser if the geometries of your features
are complicated, or if you have to draw a lot of features (that is really a
lot). I have seen that happen, that a getFeature request returns several mb
of reponse, killing the browser.

As an aside, you can also draw your own geometries on the map in a Vector
layer.

Greetings,
Edwin Commandeur

P.S. In case you are wondering: With WMS it is possible to overcome cross
browser restrictions by using the image tag in WMS layers. The src attribute
of the image tag can point at any url.

P.S. P.S. GWT and GWT-OL let you abstract from javascript and html, but that
doesn't mean you don't have to learn javascript and html (or other browser
related technologies). GWT solves many issues, such as using plain java OO
versus a javascript framework specific type of OO, compressing and
optimizing your javascript, make it possible to create modules etc. But
eventually, the code runs in the browser and it helps to know about browser
specific technologies. Especially CSS, but also javascript and html.


---------- Forwarded message ----------
From: Cameron Hunt <CH...@ci...>
Date: 2009/8/10
Subject: RE: WFS Operations
To: Edwin Commandeur <com...@gm...>


 Thanks Edwin,



I’m happy to beta test anything you want. Just to give you some background
we are just in the early stages of an in house web application to track and
present customer information via a map. We are actually very new to web
development and also Java. We’ve come from a background of Delphi and used
TatukGIS VCL component in our old app which was especially easy to use but
now we are entering a different world.



We would like to do things like shade the map based on what release of our
software customers are on. Also accessing external database information via
a common field (even if this is just manually retrieving from the DB once we
identify the associated polygon on the map). In fact all of our info will be
stored in an external database so rendering the map based on attributes will
be attributes coming from the DB so I’m hoping to be able to do things like
manually loop through the map polygons etc.



GWT-OL certainly looks like the easiest way we’ve found so far without
having learn Javascript and HTML as well.



I’m still coming to terms with understanding WMS and WFS so please forgive
me if I ask any dumb questions.



So when your new changes come through instead of creating a WMS layer I
would create a WFS layer?



Cameron


 ------------------------------

*From:* Edwin Commandeur [mailto:com...@gm...]
*Sent:* Monday, 10 August 2009 08:59 AM
*To:* gwt...@li...
*Cc:* Cameron Hunt
*Subject:* WFS Operations



Hi Cameron,

Like the WMSGetFeatureInfo, the WFS support is also new in OpenLayers 2.8.

The WFS support is high on my list to get working with GWT-OpenLayers, but
currently it is not in GWT-OL. I am just reworking the GWT-OL code and
showcase a little bit (removing warnings and deprecated AddClickListener
stuff), but after that I think the WMSGetFeatureInfo Control and the WFS
layer will be my primary goals. Having good WMS/WFS support is first on the
list, and getting a good Google Maps example a good second.

Would be nice to have you as beta tester of the WMSGetFeatureInfo and WFS
support, when I get to it. I hope so this week.

OpenLayers 2.8 does WFS requests via a cgi proxy to get past the
cross-domain restrictions. As a user you should not concern yourself with
that to much, but for your information...

Greetings,
Edwin

---------- Forwarded message ----------
From: *Cameron Hunt* <CH...@ci...>
Date: 2009/8/10
Subject: RE: [Gwt-openlayers-users] Some general beginners questions
To: Edwin Commandeur <com...@gm...>

  Thank you Edwin,



I am using Geoserver so I will look into the WFS operations. Should I be
able to perform these operations easily enough using GWT-Openlayers?



Thanks.



Regards

Cameron


 ------------------------------

*From:* Edwin Commandeur [mailto:com...@gm...]
*Sent:* Monday, 10 August 2009 07:20 AM
*To:* AbdElhadi
*Cc:* Cameron Hunt; gwt...@li...
*Subject:* Re: [Gwt-openlayers-users] Some general beginners questions



Hi Cameron,

*GetFeatureInfo*: Abdelhadi is right about the feature info on a WMS. You
would need a WMS server that supports GetFeatureInfo. GeoServer supports
GetFeatureInfo requests for example, as does MapServer. Be careful: the
GetFeatureInfo response has not been standardized by the OGC.

Since OpenLayers 2.8, OpenLayers has WMSGetFeatureInfo control that
abstracts over several different GetFeatureInfo response formats. I will add
support for that control to GWT-OL.

*Thematic mapping*: That is something I think you would have to achieve
using SLD's (Styled Layer Descriptors). Since SLD's can get quite big, in
our company we wrote a proxy for performing GetMap POST requests with custom
SLD's. That proxy is closed source however. You will have to roll your own
variant.

*Printing*: Printing is always a bit problematic in my experience. You will
probably want to build a PDF server side with the information on the map. If
the map consists only of pictures (WMS layer, Legend graphics) then you can
get quite far with some server side code and a PDF generation framework.
There is one open source printing solution that I know of, and that is the
MapPrintService in the MapFish framework. At the core it is a Java servlet
and right now I am guessing it uses some Java PDF generation library.
Printing vector features is a little more challenging.

A simple solution for printing is to provide a separate page with a clone of
the map in you app, and use the browser to print that. In my experience the
vector features get printed also, but you have little control over the
printing process itself.

Greetings,
Edwin Commandeur

2009/8/7 AbdElhadi <abd...@gm...>

Hi Cameron,

I don't think you can get the feature info from a WMS layer unless the WMS
Server supports the GetFeatureInfo. a WMS is basically just an image.

AS for Thematics, I don't think OL supports that but I am not sure.

Cheers,
-Abdelhadi

On Thu, Aug 6, 2009 at 11:34 PM, Cameron Hunt <CH...@ci...> wrote:

  Hi,



I’ve just started using GWT-Openlayers and so far it looks great. Having
said that I’ve only done a few basic things. I would very much appreciate it
if somebody could give me a couple of quick pointers in the right direction
for the following tasks please?



   - Finding out the information about an object (e.g. polygon) when user
   clicks on map (based on shapefile loaded in as WMS layer)
   - Thematic mapping (e.g. shading all polygons based on an attribute).
   - Printing. How do people provide basic printing to their users?



Thank you.



Regards,

Cameron Hunt
Software Developer
Civica Solutions Pty Limited

Ph: (07) 46393633
Fx: (07) 46393632
ch...@ci...
www.civica.com.au



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This email is from Civica Pty Limited and it, together with
any attachments, is confidential to the intended recipient(s)
and the contents may be legally privileged or contain
proprietary and private information. It is intended solely
for the person to whom it is addressed. If you are not an
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this email. If received in error, please notify the sender
and delete the message from your system immediately. Any
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not necessarily reflect the views of Civica and do not create
any legally binding rights or obligations whatsoever. Unless
otherwise pre-agreed by exchange of hard copy documents
signed by duly authorised representatives, contracts may not
be concluded on behalf of Civica by email. Please note that
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