[OpenLayers-Users] Newbie question - displaying layer of text objects on a map

[Andreas Hocevar]

Hi,

On Sat, Mar 21, 2009 at 3:58 AM, Andrew Smith <laconical at gmail.com> wrote:
> I am an absolute beginner when it comes to all things GIS.  I've been
> playing with OpenLayers for a few days and am quite impressed with what it
> can do so far.  However, something I haven't been able to find any examples
> of is showing a text object on a map.

This feature will be available in the upcoming 2.8 release. I am
currently working on making it ready for inclusion into the code base,
and it should be in the trunk in a cople of days:
http://trac.openlayers.org/ticket/1895

> Here's what I want to do. I have a GeoServer map server and a PostGIS
> database containing my map data.  In a completely separate database
> (Oracle), I have a table which has a bunch of values that are changing
> regularly.  Each value has a corresponding lat/lon value.  I want to call a
> web service from within OpenLayers which requests the text values (and
> corresponding coordinates) and then shows them as a layer on the map.  As in
> I want to see the actual text value on the map, not a marker where the text
> should be.  I would then periodically redraw the layer to get the updated
> values.
>
> I guess my questions are as follows:
>
> 1. Can you actually add a layer of text objects in OpenLayers?  If so, what
> layer type do you use?  Can anyone point me to a publicly accessible
> example?

This will be a vector layer then.

> 2. Is my approach reasonable?  I could potentially copy the data from the
> Oracle database into the PostGIS database if it would make things easier,
> but I'm still not sure how to then get the data from PostGIS into my map. I
> know that PostGIS has functions like 'AsGML' and 'AsSVG'. However, this
> leads me to more questions:

Copying the data from Oracle to PostGIS would make things much easier
if you also consider using a WMS like GeoServer or Mapserver. Both can
connect to a PostGIS data store and output your points either as WMS
(map image) or WFS (vector data to be rendered on the client). This is
the state-of-the-art solution, WMS being preferred if you have a huge
number of features.

If, however, you do not want to bother with the conversion of your
data from Oracle to PostGIS and are fine writing a small server side
script to provide your text data, I'd recommend sending GeoJSON to
OpenLayers (but only if the number of labels does not reach into three
figures):

{
    "type": "FeatureCollection",
    "features": [
        {"type":"Feature", "id":"ft_1721", "properties":{"label":
"foo"}, "geometry":{"type":"Point", "coordinates":[-89.296875,
-14.4140625]}, "crs":{"type":"OGC",
"properties":{"urn":"urn:ogc:def:crs:OGC:1.3:CRS84"}}}
    ]
}

You will be able to style this in a way that just the label and not
the point is shown. As you can see, you have a point feature here with
a label attribute. All you need to render in OpenLayers (see my
explanation on GML below, which uses the same concept).

> - I know that SVG supports text objects but don't know if OpenLayers can
> overlay SVG objects on maps.  Is SVG even a viable option?

SVG is the technology used in OpenLayers to render vector layers
(including text once this feature is ready) in all browsers except
Internet Explorer. But I would not recommend sending SVG output to the
client, since OpenLayers is not designed to display third-party SVG.

> - I know next to nothing about GML. My quick Googling tells me that GML has
> things like LineString, Polygon and Point, but I haven't found anything like
> a 'Text' element.  Is GML just for displaying vector shape objects?

GML works with features. Features have a geometry and other
attributes. To display text, you will want features with a point
geometry and a text attribute containing your label text. And there is
no need to touch GML manually if you use a WMS. You will get images
that you can display using OpenLayers.Layer.WMS.

Regards,
Andreas.

-- 
Andreas Hocevar
OpenGeo - http://opengeo.org/
Expert service straight from the developers.