[mkgmap-dev] Display of overview map at large zoom values

Sun Feb 22 14:57:42 GMT 2009

Hello,

I have noticed that when I view a map in Mapsource, and I zoom out to  
a scale of 300 km or above, the entire map disappears. The problem is  
worse on Mac OS X using Garmin RoadTrip, as RoadTrip does not display  
tile boundaries.

I found that adjusting the "topBits" attribute of  
OverviewMapDataSource.java changes this behaviour. If I set topBits =  
11, the map can be displayed at scales of up to 1500 km. The comment  
in the code above this attribute, "// TODO need to change this.",  
seems to be a hint that this is not optimal. ;-) (Also, in my tests,  
if I lowered topBits to a value below 11, the entire map would not be  
displayed; with 11 the overview map is still not displayed at scale  
values of 2000 or 3000 km. Obviously a more comprehensive solution  
would be desirable.)

- Is this a known issue? Is this restriction deliberate?

I assume for this modification to be acceptable, the style files must  
be adjusted so as to not display excessive detail at these zoom  
levels. I have already done this in my custom style files: performance  
is reasonable.

I am certainly not familiar with Java development in general, and  
definitely not with this project in particular, so I imagine that my  
modification will most likely not be suitable for general use.  
However, I have included the diff output below for your consideration  
anyway.

Your comments would be appreciated.

--- mkgmap-src/src/uk/me/parabola/mkgmap/reader/overview/ 
OverviewMapDataSource.java	2009-02-10 16:45:16.000000000 +0100
+++ mkgmap-trunk/src/uk/me/parabola/mkgmap/reader/overview/ 
OverviewMapDataSource.java	2009-02-21 18:04:15.000000000 +0100
@@ -45,7 +45,7 @@

  	// TODO need to change this.
  	private final int topLevel = 5;
-	private final int topBits = 14;
+	private final int topBits = 11;

  	/**
  	 * This is a fake source of data and is not read from a file, so  
always



