[GIS] How to prepare or normalize raw shape (.shp) files for OpenLayers2 use

mapserveropenlayers-2qgisshapefileweb-mapping

It seems like I am having difficulties in normalizing raw map files' resolutions to OpenLayer's use most probably in parameters mapext and imgext. I hope you can help me with this.

Using raw (.shp, .shx, .xml, .prj, .dbf) and associated raster files for landuse-related data included in an ArcGIS 10 project (.mxd) that I've obtained from a credible private source, I have used Mxd2Qgs to read .mxd project to .qgs. Then I used Quantum GIS' MapServer Export plugin to export selected layers to a MAP file.

I can view the generated map in browser but cannot do so when I use it through OpenLayers. The generated map file has an extent:

EXTENT 272813.696194 1546212.773835 340292.781152 1587773.606956 
# (it's quite huge).

I have tried adding to OpenLayer's map initialization properties maxResolution, numZoomLevels, units, projection and displayProjection but there is no map rendered, although the map has been queried. I have just guessed the maxResolution here, copying from other huge datasets online 🙂

The map has never been queried when I added the map file's EXTENT to maxExtent: OpenLayers.Bounds(…). The same thing goes when I used the files included in the MS4W tutorial #1 data set.

My questions would be:

How to know a given map's maxResolution parameter for OpenLayers? Can this be guessed?
Are there any special processing needed for map/shapefiles before they can be used in OpenLayers?
How should a map file be written so that it could be used in OpenLayers?

EDIT: Attached MAP file and JavaScript files.

Best Answer

You can use maxResolution: 'auto', if you dont know your maxresolution. it is how many "units" does a pixel on the screen represent.

if you want to calculate it you can use this formule: (details are here)

Units Conversion Factor
inches        1
ft           12

mi        63360
m       39,3701
km      39370,1
dd      4374754      Decimal Degrees

Map DIV               The size of the area on the webpage the map will go in.
WIDTH         600     pixel
HEIGHT        600     pixel

Units        dd      Units our map is in for OpenLayers            
Factor  4374754      Scale factor based on units.

dx         = Left - Right
Scale      = dx/WIDTH * dpi * Factor

Resolution = Scale / ( dpi * factor ) = 4000000 / ( 72 * 4374754  ) = 0.012699

with your que. about shapefile, you can use directly shapefile in openlayers with Openlayers shapefile method.

OpenLayers.Format.Shapefile

but i suggest that you should use geojson format from server or local for getting faster performans and more browser compatibility which i have been using it for a long time. You can easily convert your data to geojson with qgis. Besides this, javascript shapefile viewer is another choose. Some example pls see here and Convert Shapfiles and DBF to GeoJSON using JavaScript here.

i hope it helps you...

Related Solutions

[GIS] Problems with features, popups and strategy with OpenLayers

you can catch your opened(active) popup with this way:

map.popups[a]

you can change so update its content:

map.popups[a].contentHTML = 'Test Content';
map.popups[a].redraw();

or

map.popups[a].setContentHTML = 'Test Content'

you can show or hide your popup:

map.popups[a].show();
map.popups[a].hide();

you can update position of your popup:

map.popups[a].lonlat.lon = 10;
map.popups[a].lonlat.lat = 10;
map.popups[a].updatePosition();

for openlayers cluster you should check out Cluster Strategy Example here. Only one thing you should do is that define fillColor or externalGraphic for your cluster.

          var style = new OpenLayers.Style({
                pointRadius: "${radius}",
                fillColor: "${getColor}",
                fillOpacity: 0.8,
                strokeColor: "#cc6633",
                strokeWidth: 2,
                strokeOpacity: 0.8
            }, {
                context: {
                    radius: function(feature) {
                        return Math.min(feature.attributes.count, 7) + 3;
                    },
                    getColor: function(feature) {
                        return // Your color selection logic;
                    }
                }
            });

As the last question, i dont understand very well what you want? but try to use removeFeatures instead of destroyFeatures....

i hope it helps you...

OpenLayers – Difference Between Projection and DisplayProjection

From the OpenLayers Beginner's guide:

This is a property that is used mainly by controls which show coordinate information. By setting this property on the map object, any control that has a displayProjection property will be set to this value. Controls, such as the MousePosition control, can display coordinates in the displayProjection. So, your map could be in a different projection than what you wish to display the coordinates in. However, to use a displayProjection other than EPSG:4326 or EPSG:900913, Proj4js must be included on your page. This property comes in very handy if, for instance, your map is in a spherical Mercator projection (i.e., EPSG:900913), but you might wish to display coordinates in another projection, like EPSG:4326 (to display lon/lat coordinates).

The bold parts should give you an idea. I hope it helps.

Best Answer

Related Solutions

[GIS] Problems with features, popups and strategy with OpenLayers

OpenLayers – Difference Between Projection and DisplayProjection

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