Re: Mapsource Mileages Generated Are Not Accurate - Question - Why?

From: George Pinson (gfpinson_at_*nospam*bellsouth.net)
Date: 09/23/04 

Date: Thu, 23 Sep 2004 11:41:43 -0400

On Thu, 23 Sep 2004 02:27:25 GMT, whereami
<oaknuts@yahoo.com> wrote:

>Peter <prathman@comcast.net> wrote in
>news:l8Odnc7dKb9FFNLcRVn-rw@comcast.com:
>
>> Tommy M wrote:
>>
>>> shark572@yahoo.com (Tommy M) wrote in message
>>> news:<d759909e.0409191314.19d23a97@posting.google.com>...
>>>
>>>>I recently did a 19 leg trip using a Street Pilot III and Mapsource
>>>>6.3/City Navigator v5.0 generated routes. The total mileage of all 19
>>>>individual route legs was 2907 miles, actual mileage ended up being
>>>>3406 miles.
>>>>
>>>>A good portion of the mileage was through mountainous routes.
>>>>
>>>>Can anyone here tell me how/why this 499 mile variance occurred?
>>>>
>>>>Thank you.
>>>
>
>
>> Your total discrepancy is about 15%. As discussed before there are a
>> number of possible sources. On twisting roads the maps may not
>> include all the smaller turns and that could cut the distance up to a
>> few percent; MapSource figures horizontal distance rather than slope
>> distance and that could reduce the distance by some fraction of one
>> percent; and there could be some minor side trips that didn't get
>> figured separately. But that still seems to leave an error of at
>> least 10% and the first thing I'd check would be the odometer
>> calibration - freeway mile markers are a convenient way to do this but
>> pick an area with few exit/entrance ramps since they may relocate the
>> markers to avoid these. Next I'd suspect some map database
>> discrepancy. Comparing mileages for subsets of the trip would help
>> pin this down. Did the cursor appear to stay on the marked route line
>> at all times?
>>
>
>When I lay out motorcycle enduro routes, cutting the corners instead of
>staying on one side of the road can cut a tenth of a mile off every mile
>traveled when the roads curve a lot. That can throw the time checks way
>off.
>Randy

I normally don't use routes that follow the road as my GPS
(176C) does not do autorouting. But out of curiosity, I
turned on autorouting in mapsource. My new route (distance
know to be 343 miles) showed 378 miles. A close examination
of the route showed waypoints which were not precisely
placed and as a result, there was some doubling back in
order to go through the waypoint. In my route, a little
work and zooming way in and moving the waypoints to be right
on the road and the final rout from Mapsource was **** 343
miles ***.

As a retired computer operator/programmer/systems analyst, I
still remember one of our favorite sayings.

"Unfortunately, computers do exactly what we tell them to
do, NOT what we want them to do."

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