Flex - Styling Interactive Maps

I had an interesting question recently. This was with regards building a mapping application using the open source mapping Flex library Openscales. He wanted to improve the styling of the map layers, but still be able to click on a feature and return attributes. Now Openscales is nice since both WMS and WFS calls are easy.

Let me step back quickly and explain the difference between WFS and WMS. Both involve calls to a spatial server, in this case to Geoserver. WFS returns the raw data. So Flex is able to draw the boundaries of features, making the feature interactive. The features attributes are also returned. Thus a click on a feature can return the features attributes without a additional server call. WMS simply returns an image. Getting the attributes for the latter would require both an additional server call and constructing a new query.

It struck me after thinking about this for a while. Why not combine WFS with WMS. Thus request both a styled WMS image and draw on this the WFS features. Make the WFS feature fully transparent, thus only the WMS styling shows.



After playing with this, the results were pleasingly good. The one thing I'm finding is challenging is the ability for a selected WFS feature to be highlighted when selected. Since it is fully transparent.

I have a potential solution. And will share once I have tested

Map Technology Overview

I've been on a technology review of late. Mostly driven by client requests. I never cease to be amazed by what is now available. So far my thoughts in brief:

Mapserver is a pain to configure. Manually writing mapfiles, takes me back to the days of ArcIMS and axl files. Geoserver is far easier in every respect. It has a terrific admin interface. Setting up basic map services, WMS, WFS is easy. Exporting in formats read by google maps etc is also a piece of cake. If it were not for the need of a full application server, the choice of geospatial server would be easy. Geoserver also allows for easy generation of Georss, KML and viewing of map layers in openlayers.

Caching and tiling is another area which favours Geoserver. Geowebcache is integrated into geoserver. No need for additional work. The config screen allows for easy seeding. Mapserver works with tilecache. From the work I have done so far you tile mapserver by calling tilecache and passing the mapserver url.

I'm still working with openlayers but am so far very impressed. One thing I already notice is how much is there. I really like the Modest Maps Flex library. But with a smaller community, it does not have the same depth of code. I'm still playing here, so more feedback later.

I'd like to know one of the big fours - mapquest, MSN, Yahoo and Google - API's. I lean towards mapquest, largely due to the feedback I have read and my impressions from conferences. I recently signed up for their developer account. A couple of people were soon emailing me with contact info. On one side I thought this great, since support might be more forthcoming. On the other side, money reared its ugly head. Use of this service beyond local testing has a price tag! Reminds me why I so favour open source.

Other areas on my radar at the moment beyond testing the mapquest API include:
- Geodjango testing
- PostGIS and shapefile write up
- Javascript to Flex/Flash communication

I'll write more on this in due course. I asked Paul Ramsey (OpenGeo and PostGIS author) his thoughts on Geoserver versus Mapserver. Here is his repy:

If you are deep into OGC standards, Geoserver has a big advantage in
completeness of implementations of things like SLD and WFS. (Less on
WMS).

I did a talk last year which, though tongue-in-cheek addresses the
more important differences. Which are philosophical more than anything
else. Scriptable web rendering engine vs complete spatial web
environment.

http://s3.cleverelephant.ca/geoweb-mapserver.pdf

Most of the speed differences are gone at this point, they perform
very comparably for common use cases.

GeoServer and WMS

Geoserver is a truly fabulous open source geospatial server. Much easier to set and configure than Mapserver. The documentation provided is extremely comprehensive. See:

http://docs.geoserver.org/1.7.6/user/getting-started/web-admin-quickstart/index.html

Here are the notes I made as I walked through setting things up.

a) Setting up geoserver

I am using the portable gis control panel (see earlier blog entry). Once installed it sits directly under c:/.When you open it you have the option (under web modules) to start Geoserver.
Access Geoserver via:

http://localhost:8080/geoserver

admin/password are the default credentials. If you need to change them look at:

C:\Program Files\GeoServer 1.7.6\data_dir\security\user.properties

b) Adding shapefile

Under the data directory for geoserver add some new data in the data directory:

C:\Program Files\GeoServer 1.7.6\data_dir\data\

In the web interface click on data -> datastores -> new. Add name and type. Next screen add path to data and data name eg. nyc_roads.shp. Now click submit. The next screen should be the featuretype editor. Now from drop down select the type eg. line add the srs, then click generate. Now click submit and finally apply. Note these two steps in this order are critical in the process.

In the web interface under data->datastores you should now see your new data

You can view the data in welcome->demo->map preview

c) WMS

In config screen choose wms. Next choose contents and add the layer you just created in this screen. You can get the SRS info from the featuretype editor under data

One of the amazingly cool things about geoserver is the demos page in the web interface. From here you can view the results of various requests. Choose sample requests to see these options; wms, wfs etc. Also from here you can select map preview. From here you can see your newly loaded data. Another terrific thing about geoserver is the integration of openlayers. Data can be viewed directly in the geoserver interface with openlayers

d) Geowebcache

Another terrific part of geoserver is the integration of geowebcache. There is a nice guide to this here:
http://geoserver.org/display/GEOSDOC/GeoServer+GeoWebCache+Configuration

Key directory is C:\Program Files\GeoServer 1.7.6\data_dir\gwc. Go to:

http://localhost:8080/geoserver/gwc/demo


Here you can see all of your layers and ‘seed this layer’. Seeding is generating tiles. When you select seed this layer you will see under the gwc directory your newly generated tiles.
If you go to the demo page and view your data. In openlayers you have the option to view the data as a single image or tiled. The latter will show your newly generated tiles.

If you wish to see the data in google earth. Open Google earth, choose add->network link. Add a name and the url below to the pop up.

http://localhost:8080/geoserver/gwc/service/kml/topp:nyc_roads.png.kml

By selecting the named link in google earth (under my places) you should see your data.