How is a GPS Garmin "tunnel aware" ?

**Cmagalha** · 23rd October 2012, 01:29 AM

My GPS 3790T is tunnel aware.

When I enter a tunnel, it turns the screen into a dark background, like night mode.

Question is: HOW ?

Does it has a light sensor somewhere ?

**Giomen** · 23rd October 2012, 02:21 AM

It is one of the last nice features on Garmin maps. Coordinates are plotted for tunnels and big underground overpasses sector. When you is driving in this sector the following events occur:
- switching in night mode as you notice
- simulation driving with speed what was before your device lose communication with satellites (GPS usually not work under ground if there are not GPS repeaters) !!!Be careful - you may stop in tunnels but vehicle icon will be move across the screen as long as the Nuvi does not understand that it was deceived (I often seen this picture in traffic jams in big underground overpasses)!!!

**Roger** · 23rd October 2012, 06:59 AM

Originally Posted by Giomen

.... Coordinates are plotted for tunnels and big underground overpasses sector....

I do not think it's a matter of coordinates, but simply a specification of the TYP file (my personal thoughts)

If you notice, you'll see that the color of the road for the section of the tunnel is different from the color of the normal road.
And being the vector map, in fact bears with him the specifics of that stretch of road : will be recognized as all that stretch of road tunnel

Bye,bye

**Giomen** · 23rd October 2012, 08:55 PM

Originally Posted by roger1109

I do not think it's a matter of coordinates, but simply a specification of the TYP file (my personal thoughts)

If you notice, you'll see that the color of the road for the section of the tunnel is different from the color of the normal road.
And being the vector map, in fact bears with him the specifics of that stretch of road : will be recognized as all that stretch of road tunnel

Bye,bye

We bet it was before eggs or chicken?!

Vector map it is coordinates inside only! No coordinates of the specifics of that stretch of road then Nuvi after losing satellites posted a question mark, stop navigation and begins a nightmare for you. It is often occurs on underground overpasses with many traffic lane even now. Garmin maps is not perfection as we know

That's what I meant only...

**Roger** · 23rd October 2012, 09:10 PM

Originally Posted by Giomen

We bet it was before eggs or chicken?!

Vector map it is coordinates inside only!......

Bohh??!!! I'm not part of that race of beings, for I know

I think the correct answer (which is what I wanted to say, but I apologize for the difficulty of my English) and that of post #4

Bye,bye

**Giomen** · 23rd October 2012, 11:54 PM

Originally Posted by roger1109

I think the correct answer (which is what I wanted to say, but I apologize for the difficulty of my English) and that of post #4

Whether there is difference between my post and post #4? Vector map contain only digit coordinate of points to which attributes attached. All what we can see on screen it is a plot of the points.

**Cnfhbr** · 23rd October 2012, 08:45 AM

It's quite simple. This road segment (tunnel) has a special attribute in the map - IS_TUNNEL (1 or Y in this case), that activates the interpolation mode of the unit during the possible absence of a GPS signal (with all that it implies ...)

**Cmagalha** · 23rd October 2012, 01:33 PM

Thank you all.

I am using the brazilian map "Autoguia", not a Garmin map, but the behavior is the same.

Very good answers !

**Cnfhbr** · 23rd October 2012, 02:45 PM

Originally Posted by Cmagalha

I am using the brazilian map "Autoguia", not a Garmin map, but the behavior is the same.

The 'Autoguia gps' maps are created by means of MPC compiler which supports this IS_TUNNEL attribute in the shapefile of roads, so the effect is the same ...