[mkgmap-dev] max-speed and arbitrary values

Wed Jun 3 18:20:51 BST 2009

Hi Felix,

Am 03.06.2009 um 12:06 schrieb Felix Hartmann:

> Is it possible to encode arbitrary maxspeed values or can we only  
> set in steps of 10km/h?
>
> For example having road  road_speed=7 associated with 35km/h, road  
> road_speed=6 with 27, road_speed=5 with 23, road_speed=4 with 20,  
> road_speed=3 with 17, road_speed=2 with 10 and road_speed=1 with 5km/ 
> h. The difference this should make would be enough to only set  
> road_class=2, 1 and 0 and avoid the big time penalties for sharp  
> turns that happen in road_class 4 and 3.
>
> This would be great for bicycle maps.

That would be great indeed.

> In Mapsource one can change the speed oneself, I noticed that  
> dividing default speeds by a factor of 3.5 produces pretty good  
> estimation of arrival times for bicylces (when using the car/ 
> motorcycle setting, as bicycle produces rubbish routes) but on the  
> GPS this is not possible.
>
> @Thilo, do you understand the code good enough to write a patch for  
> this if possible.? I have problems understanding in which files the  
> maxspeed is handled.

I have not really looked into the Garmin encoding part of the code,  
but I could invest some time if it helps. From what I know there are  
only those few road_speeds available in the format. *But* maybe there  
is some part in the header where one can set what actual speed each  
road_speed corresponds to (this is pure speculation).

Somebody mentioned also that the GPS units will "learn" the speed you  
are actually driving and use that for their calculation. If this is  
speculation or based on facts I don't know. At least with my Oregon  
300 I doubt it: As I use it all the time with my maps I'm cycling  
always in the car mode. So far the ETAs are still very wrong. If the  
GPS would learn the speed they should become more realistic over time.

Perhaps we can build a Wiki page somewhere where we can collect all  
"hard evidence" about the routing? How about setting up an artificial  
map that we can use to test the routing, ETAs and so on? Especially  
keeping in mind that there might be a difference between different GPS  
units, firmware revision and so on. If there are routing parameters  
that the GPS units "learn" about their users that would really f*ck up  
our tests.

@Marco Certelli: You've already started some tests. If you could write  
down what you did on some Wiki page so that others can repeat your  
tests that would be really helpful.

Regards
Thilo