map performance tuning


I am build rather bigger maps 0.6 Gb and sometimes even 1.3 Gb img. My unit (garmin 78s) loads and zooms these bigger files slower than smaller so I started to be interested in some ways how to optimize performance of my map.
But woudn’t like to sacrifice details an quality in close zooming.
My guts saying that maybe splitter’s --max-nodes option could help.

Maybe some other tricks exists?

I also noticed that on “mkgmap-dev” going discussion about overview branch … can overview be used in unit map and can improve map performance?

Thanks for suggestions


I would start with the overview branch.

In your options file add something like

levels = 0:24, 1:22, 2:20, 3:18
overview-levels = 4:17, 5:15, 6:14

Everything from level 4 is left out from the GPS unit and only visible in Mapsource.
At these lower zoomlevels the unit uses its own background map, so the loading goes much faster.
So on your gps you see only levels 0-3 which makes it lighter.

I just this options file on my biggest map:

levels = 0:24, 1:23, 2:22, 3:21, 4:20
overview-levels = 5:18, 6:16, 7:14

before it was:

levels = 0:24, 1:23, 2:22, 3:21, 4:20 , 5:18, 6:16, 7:14

So only last three lever are overvie=levels instead of normal levels.

My gmapsupp.img didn’t changed a size at all is it ok? I expected some size reduction.
I didn’t checked performance in unit yet.



I thought the same. But I had some maps with sizes larger than 1 GB. And in this maps i had some small areas without informations. And with tests in which I tried to vary the max nodes option I found out that this deficiencies dissapear when I reduce the Max-nodes considerably. So, in case of this large maps, to increase the max-nodes, may speed up, but it creates also a deficiency.

Thank you for sharing experience … With my 1.3 GB map I had to go with max-nodes=450000 with values 600000 I was getting errors from mkgmap when building (when trying mkgmap overview branch).
With stable branch 2620 I can go 600000 but not higher … I think these are quite small numbers compared to default 1600000.
On which value max-nodes you was able to avoid deficiencies?


I finally tested in unit … Yes only “levels” are visible in unit … interesting is that gmapsupp.img stayed in same size 1.3GB but maybe removing first 3 levels (which dont have too much information) just did’nt make big difference in size.

Which background map you mean? Garmin’s Basemap? I didn’t saw any map when have been at “overview-levels” … just one color background.

Maybe I am missing something?

I build map as usually, no extra option:
java -ea -Xmx6144m -jar mkgmap-overview2-r2628.jar --precomp-sea=xx --bounds=xx --style-file=xx -c arguments xx/6324*.osm.pbf my.TYP

maybe these extra ovm_xxx.img files wasn’t included in my gmapsupp.img file?


The basemap on your GPS, maybe switch it on?
The ovm_xxx.img are only temp. files to create the basemap on your pc, the gps cant read them.

I tried this before, but than i got basemap only. Basemap didn’t switched to “mymap” when I was zooming in.
I will give another try at the evening.


I tried with another map (much smaller) and different mkgmap …before 2628 now 2629 and it is working now as expected with basemap switched on.