0

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.

  • What .NET web framework are you using? – yxcv Nov 13 '15 at 7:55
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);


}
0

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.

0

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);
  • No need to do that - ASP.NET does that for you if you do it properly. encosia.com/… – mlinth Nov 13 '15 at 11:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.