Sunday, October 01, 2006

Distances - not always as straightforward as you'd think

So I started putting distance table information in, and hit a snag along the Ashton when it started objecting to discrepencies between the distance table and the calculated distances.

Using a convenient Google Maps pedometer I crossed checked the distances.

For the fist 5 or 6 places it was really good:


PlaceDistance tablePedometer
Ducie Street, Manchester0.00.0
Lock No 1 Ancoats Junction.40.391
Lock No 3 Ancoats.50.495
Lock No 4 Bewicks Locks1.31.294
Lock No 8 Clayton Bottom Lock2.02.036
Clayton Junction2.42.485
Clayton Lock No 132.82.727

The last couple of these are showing slightly bigger errors, but it's still all well within what is needed for canal journeys. In fact, for Clayton Junction the data table merges it with Clayton Top Lock, which is a bit earlier, so the extra on the pedometer is explicable.
But now it starts to go wrong:
Clayton Top Lock No 163.53.033
We're out by nearly half a mile. Looking at any map it's clear that Clayton lock 13 is nowhere near that far from Clayton Junction. By the time we get to the end of this stretch, we're back on track
Fairfield Junction3.83.735


So the moral seems to be not to trust the distance tables explicitly. It also shows that the pedometer approach may well work for getting pretty exact distances for all places. Now I'm not doing that for all 7000 places, but it could get build into the lengthsman feature at some stage.

In passing, the overall discrepency between the pedometer and the distance table is 128 yards. At 3mph this is less than 1½ minutes! I can live with that.

1 Comments:

At 11:44 PM, Anonymous midnight cowboy said...

Nick

You did it again!! I hadn't even heard of google maps pedometer - great!

look forward to using it when i go out as a lengthsman

 

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