mobile and gis dev notes

Summary
One of the upcoming power features of Mapserver 5.0 is its AGG Rendering engine. Above is an example image of the AGG rendering capabilities of Mapserver. Below is a list of my experiences with Mapserer 5.0 AGG

• Slower than GD ofcourse, but cartographic quality is really impressive…
• You need to have SUPPORT=AGG to be able to use AGG. So far, I was not able to compile mapserver source with AGG, however using mapserver-beta-3 for linux and ms4w betas worked fine.
• Layer Transparency. I was not able to get transparency for my roads overlaying on top of my basemap. Tbonfort from IRC suggested I use RGBA for the outputformat, but it still breaks. Hope Im not missing anything important here…

The Mapfile
Currently, the mapfile ( map_all_agg.map ) contains 2,587 lines! The lengthy mapfile consists of the ff: districts or political boundaries, water bodies, greens or open spaces, roads, subway lines, subway stops. The roads is classified into 12 categories ( Expressway, Class-1, Class-2, Class-3, Class-4, Major Road, Main, Secondary, Minor, CommunityRoad(w/name), CommunityRoad, ParkRoads). Class1-4 is a classification for Roads outside the 5th Ring Road of Beijing. While Major Roads – Minor Roads is a classification inside the 5th Ring. The roads were drawn as a line layer and not as a polygon.

How to achieve the overlapping of road intersections?
The technique is to seperate the road boundary layer from the actual road line. A mixture of widths and color management was employed to achieve the effect. Essentially, we started with the ff:

1. road boundary layer
2. road layer

Further , trial and error resulted in refining the road boundary layer with different minscales and maxscales. The same was adapted with the road layer. Download map_all_agg.txt. My next iteration, would be to refactor some of the road layers if possible.

A couple of notes to myself, we could speed things up by trying out the ff:

• Using SHPTREE
• Simplifying the geometry as you change scales

Testing Methodology
1. Use Firefox and install firebug.
2. Download YSlow from Yahoo.
3. View load times using “net” tab

Using SHPTREE
1. The utility specifies indexes as it slices the shapefile into quadrants as specified from its definition. To run shptree, do the ff:

$ shptree roads.shp
creating index of new LSB format


2. It would generate a roads.qix. No changes need to be made with the mapfile. Mapserver loads the shapefile without the extension.

3. Testing… Significant speed improvements were made as shown below:

• Before: 29.44 secs
• After: 5.91 secs