Calculating slope angle witha slope angle card

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 norrisdan71 03 Feb 2023

Afternoon, forgive a daft question, it is Friday afternoon.

I got a slope angle card with my new Silva compass: it's for 10m contours on 1:50K maps - I'm using 1:50K maps with 20m contours. Given that the contours measure double the 'rise' for the same vertical 'run' do I just double the angle which the card measures (eg the cards says its a 20degree slope which interpret as a 40 degree slope) when the lines are aligned with the contours on the map? Thanks 

In reply to norrisdan71:

Are you sure your 1:50,000 map contour interval is 20 metres? I’ve never seen that interval, all my maps of that scale are 10 metre interval contours.

In reply to phizz4:

I thought that but I've seen some OS map series with odd contour intervals (like 5 or 20) so *assuming* it's a UK map, still possible.

 Mark Bull 03 Feb 2023
In reply to norrisdan71:

Doubling the angle is roughly right for small slopes, but the error gets bigger as the slope gets steeper. If the card has a scale for 10m contours for 1:25k you can use that instead! 

 cdpej 03 Feb 2023
In reply to norrisdan71:

The Eagle ski club produce a very good angle card (its gone through a few iterations over the years) and covers most common scales and contour intervals.  I can post you one if you message me.  They have become much less useful since most countries produce angles on their maps.  For those that dont you can get angles off Fatmap (now strava!).  Interestingly snow on the ground can significantly change angles calculated off the underlying ground.  

OP norrisdan71 04 Feb 2023
In reply to cdpej:

I dble checked: 20 m intevals 1:50K maps covering Hardangervida in Norway from Nordeca. Thanks for the advice. I've messaged you cdpej - possibly twice.

In reply to norrisdan71:

I wrote a tool some years ago, to create slope cards for any scale, contour spacing and range of angles of interest. You just change a few values at the top of the PostScript file, and print it.

I also combined it with a vernier Romer tool, and protractor, again, configurable.

Drop me a message if interested.

Post edited at 13:47
OP norrisdan71 05 Feb 2023
In reply to captain paranoia: thanks, I’ve messaged you

In reply to norrisdan71:

Did you get the reply?

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