Navigation question

[Hogan]

I was wondering, just how accurate are the OS Explorer maps? The reason I ask is that when I take a bearing using a Silva Ranger 15TDCL compass (with the declination adjustment set as per map guide) on my local church spire (about 1/2 mile away) from my home, when I transfer reading to map, it is spot on. As is the reading / bearing to the top of a hill nearby (about 2 miles). ....so what is the problem I hear you ask?

When I do exactly the same, with a transmitter tower about a mile away, it is way out. Unless someone has moved the transmitter 1/2 mile down the road , (unlikely, but anything's possible I suppose!) - the only thing I can think of is that the map is wrong.

I get exactly the same degree of error when using a different compass and manually adjusting for declination.

Its the only transmitter anywhere in the area, so I can't be mistaking it for another one nearby.

I hate to think of the map being wrong. I don't like to think that in a potential survival situation, that the map is nothing more than a rough guide to where things are, especially the renowned OS Explorer maps. Perhaps I'm being over-fussy, or my map reading / compass skills need work, or I am looking for too much precision, or maybe there's a much simpler solution that I'm just not seeing (the 'doh' factor).

So over to you, any ideas? or similar experiences?
Thanks in advance

[Adi007]

How much is "way out?"

[Wayne]

Occasionally the OS makes a mistake. it may be worth contacting them about your findings.

i was solo backpacking in North wales once on a marked path on the map. i was using a power line as a handrail so i knew i was in the right place. however i refused to believe the map might not be right even when i was chest deep in a bog. i carried on regardless and eventually found a route through. it was a warm day and i had plenty of gear with me so i didnt mind the adventure. however in poor conditions it might have been serious.

That aside we in the UK have great maps. When i travelled in Alaska i was shocked at the poor quality maps. things might have improved there but in foreign climes your be hard pressed to beat the OS.

[Hogan]

How much is "way out?"

Transmitter is about a mile away. Reading shows it to be about 1/2 mile left of where it actually is.

[RAPPLEBY2000]

in the last 8 months of mountineering and training with MIA standard tutors they all said the only features on any map that can be 100% trusted are the contour lines!

forests pop up and are cut down,
buildings and structures can be newer than the map or just have been destroyed.

rivers burst banks and dissapear in summer.

paths get eroded, get bigger, get overgrown and turn to streams in rain!

contours are the only reliable thing on a map!



one last thought, are you adding/subtracting the same amount of declination when looking at both objects? (obvious but an easy mistake!)

[Hogan]

one last thought, are you adding/subtracting the same amount of declination when looking at both objects? (obvious but an easy mistake!)

Yeah, I think so... using the Ranger 15TDCL there is a 'declination adjustment' screw which keeps the declination constant without having to do the mental arithmetic each time. Also using the standard Silva Type 4 and performing the declination adjustment manually, it is still only that one transmitter that appears to be mobile. Everything else is where the map says it should be.

Maybe I should start a campaign to have contour lines marked on the ground (using eco-friendly materials of course)

Regards,

[Hogan]

in the last 8 months of mountineering and training with MIA standard tutors they all said the only features on any map that can be 100% trusted are the contour lines!
contours are the only reliable thing on a map!

With all the landslides / floods recently, is there any truth in the rumour that OS Maps will in future be done in pencil or made with 'fuzzy felt'? :wink:

[masongary44]

A while ago while training as part of a team for an adventure race, we did a quick exercise where we aligned a piece of string to nrth using one compas, then placed our individual compasses on the piece of string to compare...
The accuracy ranged through (about) +/- 4 degrees, depending on how old the compas was, the quality of the compas and where it has been kept for the last 5 years..

So I suppose another good question would be, is my compass as accurate as the map I am using??

(Incidentalyy, mine was one of the worst... )

[Tedders]

I was told, though I can't remember who by now, that OS build in mistakes on purpose. In this way they can tell if people break the copyright laws by copying it or reproducing it as their own. I would have thought that they would only do it with minor features not towers.

[Roving Rich]

The other thing to bear in mind is magnetic deviation changes, so is different depending on tha age of the map and location.
Just a thought

Rich

[Adi007]

I was told, though I can't remember who by now, that OS build in mistakes on purpose. In this way they can tell if people break the copyright laws by copying it or reproducing it as their own. I would have thought that they would only do it with minor features not towers.

There are errors but I doubt that they are deliberate ... they contain plenty of minor errors. Also, add to that the fact that no two survey is going to be the same so any map that looks like an OS map is going to be an OS map or a copy.

[sargey]

it might be worth checking the map age and the transmitter age. it could well be that one transmitter has been knocked down and another built. specially mobile phone masts.

round here there are several roads that don't show up on the 1:50,000 maps but do show up on the 1:25,000 maps. it's simply due to the fact that the 1:50,000 scale maps are based on the previous survey, the road wasn't built back then.

cheers, and.

[chris]

Tedders

In the past ordnance survey successfully sued one of the major motoring organisations for copying and reproducing their maps, they caught them by putting in some unique markings that meant nothing whatsoever but, bang, caught red handed. :biggthump

Chris.