Archive for the ‘WMS’ Category

Bending the Rules Again

October 17th, 2008 by John Fletcher

In my last post I mentioned a way to get distinct values for a featureclass out of ArcIMS. This involved bending the rules ‘creatively’, but it got the job done which was the ultimate intent. Recently I was discussing with some colleagues another frustrating issue with ArcIMS; namely, that of stacking an ArcIMS image on top of (or below) a WMS image. Since ArcIMS has no native WMS client capabilities, we’re always forced to stack the images in the browser. This can have some unfortunate side effects – partially transparent polygons become opaque, and antialiasing goes straight out the window. Observe:

wms

This is the standard Geocortex IMF demonstration site, with the USGS Shaded Relief WMS dropped behind. Notice the absence of image behind the jurisdiction layer, the poor antialiasing of the roads over the shaded relief, and the barely legible “Gaston” label.

Now, we can’t make ArcIMS become a WMS client, but we *can* work around these issues if absolutely necessary. What happens from the IMF perspective when we generate this image is that it requests the WMS image from USGS, and at the same time requests the ArcIMS image. It then drapes one over the other in the user’s browser. If we changed the workflow a bit, we can do the following:

  1. Request the WMS image
  2. Save the WMS image somewhere the ArcIMS server can get it
  3. Add an acetate layer to the ArcIMS map with a single polygon. This polygon will cover the same area as the current map extent, which will also be the same area covered by the WMS image.
  4. Use a RASTERFILLSYMBOL to paint the polygon, and use the WMS image as the source.

Using this trick, our image now will look like this:

merged2

Ahhhh, much better!

Note that I did take the liberty of changing the symbology slightly for the jurisdictions layer to work well with the WMS, but you get the idea :)

Posted in ArcIMS, IMF, WMS | No Comments »

Thinking about cartography and WMS layers

September 15th, 2007 by jade

Web Map Service (WMS) layers allow organizations to incorporate spatial data from other places, and clients are now routinely integrating WMS layers into their ESRI-centric applications. I’m going to discuss a couple of the cartographic issues that can crop up with this.

When a user switches on a WMS layer, the map viewer requests the data directly from the WMS which returns a map image back to the user’s web browser. The map viewer stacks the various image layers from the different data sources one on top of the other. Normally this would result in only the top image being visible to the user, but the map images can take advantage of the concept of transparency; that is, areas of the map image that contain no data can be made transparent, allowing the images lower in the stack to show through.
While the image stacking method mentioned above works is simple enough, there are situations where it yields unsuitable cartographic results. Annotation can be especially problematic, as a feature’s label may have been placed at exactly the same spot as the label for a different feature from a different WMS data source. Since the WMS layers are raster images, there is no way for different WMS servers to perform label collision detection and reconciliation, which is done for WMS layers drawn from the same data source. Multiple labels drawn at exactly the same location will be illegible.

In addition to the annotation problem, there is a potentially more important issue whereby polygons (district boundaries, for example) that are symbolized with a solid fill color are completely opaque to WMS layers from data sources further down on the image stack. A common internet cartography technique is to symbolize a polygon with a partially transparent fill color, allowing features underneath the polygon to show through. For example, a floodplain feature may be symbolized with a partial transparent fill, allowing all of the land features to be seen through the feature. The end result is a tinge of color across the floodplain area with little effect on the visibility of the coincident features. However, output map images compiled from different sources don’t provide transparency options that are well suited for web environments.
So… when integrating WMS layers from various sources into your mapping application, it is important to consider how they will appear to end users. WMS layers can be a convenient way to enrich an ArcIMS/ArcGIS Server application, but are of diminished value if cartographic representation issues detract from the effectiveness of the final map.

Posted in ESRI, WMS | No Comments »