Pass GeoJSON from .NET to Javascript

Question

My .NET/OpenLayers/GeoServer/PostGres-PostGIS application allows users to create custom queries to get data from my PostGIS tables.

The queries are called in .NET (not javascript) using a web method using the following SQL query:

SELECT "tract_name", ST_AsGeoJSON("geom") FROM a_postgis_table;

The query works fine when I run it in the PostGres SQL window.

How can I pass the PostGIS results, including the GeoJSON column, back to javascript/openlayers from the .NET web method so that OpenLayers can render the vector layer in the map? There has to something really simple I am missing here. Seems like this should be an extremely common thing that people need to do.

• yxcv: What .NET web framework are you using?

Answer 1

I think there are three parts to getting your solution.

1. Creating a GeoJSON FeatureArray
2. Returing the FeatureArray from .NET
3. Consuming that on the client

1) GeoJSON FeatureArray. Easiest way (IMHO) is to write a Postgresql query to do that. There's a guide here on how to do that, but, assuming tract_name is unique your query could be something like this:

SELECT row_to_json(fc)
 FROM ( SELECT 'FeatureCollection' As type, array_to_json(array_agg(f)) As features
 FROM (SELECT 'Feature' As type
 , ST_AsGeoJSON(lg.geom)::json As geometry
 , row_to_json(lp) As properties
 FROM a_postgis_table As lg 
 INNER JOIN (SELECT tract_name FROM a_postgis_table) As lp 
 ON lg.tract_name = lp.tract_name ) As f ) As fc;

2) .NET service There are lots of ways to do this, but I have done this in the past using WCF / svc if I've got my terminology correct. Here's a little VB sketch:

 <OperationContract()> _
 <WebInvoke(Method:="POST", ResponseFormat:=WebMessageFormat.Json, 
BodyStyle:=WebMessageBodyStyle.Bare, UriTemplate:="GetTracts")> _
 Public Function GetTracts() As Stream
 ' Code to get your Postgresql Feature set into a string
 dim json as String = "" ' The sql output
 WebOperationContext.Current.OutgoingResponse.ContentType = "application/json; charset=utf-8"
 Dim ms As MemoryStream = New MemoryStream(Encoding.UTF8.GetBytes(json))
 Return ms
 End Function

3) Get it into OpenLayers ( works for v 2)

I assume you have created an empty vector layer in OpenLayers called "myTracts". You can do something like this:

 var request = OpenLayers.Request.POST({
 url: serviceUrl,
 data: '{}',
 headers: {
 // contentType: "text/plain"
 "Content-Type": "application/json; charset=utf-8"
 },
 callback: onGeoJsonLoadSuccess
 });
 function onGeoJsonLoadSuccess(data) {
// Assuming your server data are in lon/lat
 var geojson_format = new OpenLayers.Format.GeoJSON({
 'internalProjection': map.baseLayer.projection,
 'externalProjection': new OpenLayers.Projection("EPSG:4326")
 });
// You might want to empty the features first...
myTracts.addFeatures(geojson_format.read(data);
}

Answer 2

I am not familiar with .NET, but if you are able to modify the DOM from the .NET method you could write the data to a hidden div on which a change listener is registered. eg using jQuery:

$.("#data").bind('contentchanged', function() {
 //do something with data
)};

When the event fires, the function gets triggered and you can handle the data. Another way would be to write the data to a file and load it using AJAX. For both methods you need to modify the DOM, though, as you need to let JavaScript now what to load.

Answer 3

You can use JavascriptSerializer to convert your .Net list to JSON or GeoJson.

GeoJsonList refers to list in data

var serializer = new JavaScriptSerializer();
var serializedResult = serializer.Serialize(GeoJsonList);

• mlinth: No need to do that - ASP.NET does that for you if you do it properly. encosia.com/…