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Eta Off Road


Sputnik 57

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I've been using a 60C for over a year, and have found that the ETA to a point that I am driving to is reasonably accurate if there is no traffic. From what I understand, the unit uses road speed, not actual speed, to compute your ETA.

 

Last week I flew to Dallas and San Antonio, and was using my GPSr on Southwest Airline flights. Since we were in heavy clouds, I couldn't pursue my usual pasttime of identifying ground features from the map, and out of boredom, I started watching the Time to Destination reading, comparing to my our actual speed. I took a few readings on the Dallas flight, and then, flying home from San Antonio, I took more readings. The flight is pretty short from the time of the "OK to use PEDs" announcement to the "discontinue and store all PEDs" announcement.

 

Based upon my distance to Hobby airport and the unit's estimated travel time, I computed the assumed speed and compared it to my actual speedHere are my findings:

 

=====================================================

Time to Waypoint 25 20 18 15 12 10 9 6 5

Distance to Wpt 164 138 128 118 87.9 63.6 53.4 31.2 27.0

Actual Speed 460 510 523 541 501 436 399 346 340

Assumed Speed 394 414 427 472 440 382 356 312 324

 

(I know the table is hard to read since the forum strips extra spaces--sorry about that).

 

From the data, you would think that an average speed is being used. For example, from 18 to 15 minutes out, the actual ground speed went from 523 to 541 (we must have had a tail wind--I rarely see speeds much over 505 or so). The "assumed speed" changed from 427 to 472. When the actual goes to 501 at 12 minutes out, the "assumed speed" reduces to 440. But look at the last two entries. Speed declines from 346 to 340, but the "assumed speed" goes from 312 to 324.

 

Any ideas? Or should I just stick to the in-flight magazine and forget about this?

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If you are routing off road, it looks at the closing speed to the destination and calculates the time it takes at that speed. There is a running averaging, so it takes time to adjust to different speed.

 

For on road routing, it looks at what it has in the database for the travelling speeds of the roads and completely ignores how fast you are moving.

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For on road routing, it looks at what it has in the database for the travelling speeds of the roads and completely ignores how fast you are moving.

 

Tell me if I'm wrong but last summer, coming back from New Orleans, I had my Legend C on and it seemed that the faster I was going, I could see the ETA change on my screen. If it only considers the speed in the database, what happens if you stop moving for an hour, surely the ETA will change!!

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For on road routing, it looks at what it has in the database for the travelling speeds of the roads and completely ignores how fast you are moving.

 

Tell me if I'm wrong but last summer, coming back from New Orleans, I had my Legend C on and it seemed that the faster I was going, I could see the ETA change on my screen. If it only considers the speed in the database, what happens if you stop moving for an hour, surely the ETA will change!!

Of course the ETA will take into account if you're not moving, and it will change as you drive faster. But what it seems to be doing to me is what Red90 says - it takes your current location, and guesses at a speed for the remainder of the trip depending on the road database (and also your mode of transportation - on road routing with bicycle seems to generate much slower speeds for ETAs). Let me give a more concrete example: If you have ~ 1 hour left to go, and stop for a 1 hour lunch break, with on-road routing when you start driving again, it will still think you have 1 hour left to drive. With off-road routing, it averages your "closing" speed over some time period (not sure what), and that 1 hour lunch break will probably play havoc with it, and give you a very much laterETA when you first start out again. I've had it tell me (off-road routing for a driving trip) that I would get home in about 2 days, for a 20 mile, 30 minute trip. That changed a lot once I got out of the parking lot and started heading in the right direction.

 

Keith

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Tell me if I'm wrong but last summer, coming back from New Orleans, I had my Legend C on and it seemed that the faster I was going, I could see the ETA change on my screen. If it only considers the speed in the database, what happens if you stop moving for an hour, surely the ETA will change!!

For autorouting, it looks at curretn time plus remianing estimated time to get there based on the speeds it has for the roads.

 

So if you stop for an hour, the ETA increases by an hour. And yes, it uses different speeds depending on the vehicle choice.

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