Aug 3, 2021
5 Things To Check On Your Map Before Heading Out On The Trail
A top tip for your Land Navigation training
Before heading out on the trail I’ll always tell my students these 5 things to remember checking before going anywhere. I have a military background and like most things taught in the military, there’s always a handy mnemonic to help you get by. Granted, this one is not that great but anything to help you remember is good. This mnemonic is, DVAGS. (Told you it wasn’t that great) So what does it stand for?
- Datum
- Vertical Interval
- Age
- Grid Magnetic Angle
- Scale
I’ll talk more in detail about each one of these subjects.
Datum
This is quite a technical point to get across. All you need to know is the planet is not round. (It’s also not flat!) It’s known as a ‘Spheroid’. To produce a suitable ‘one size fits all’ reference point for the whole planet is quite difficult. As it’s not flat there will always be discrepancies where the land mass changes in elevation around the planet.
You have world datums and local datums. An example of a world datum would be ‘WGS84’ and a local datum would be our own ‘OSGB36’ here in the UK. A local datum will always be a better fit than using a world datum.
The important thing is to know what datum your map is working on. If you’re using a GPS, including using mobile phone apps, then the GPS unit NEEDS to be set up on the same datum as the map you are using to navigate. If they’re not then the two will not relate to each other. You will have issues with accuracy which could lead to some big problems.
Vertical Interval
This relates to the spacing between each contour line and this can vary depending on the map you are using so it’s always handy to check the map properly to find out. I’m a big user of maps produced by Ordnance Survey here in the UK. Their maps work on a 5 m or 10 m vertical interval depending on whether you’re in the lowlands or highlands (generally speaking). However, the Scottish map makers, Harvey, produce maps with a 15 m vertical interval. Always worth knowing as it helps to build that mental picture when you’re out walking the trails. You don’t want that hill to be twice as big as you thought it was going to be!
Age
All maps here in the UK will have a production date printed somewhere on it. Always try to use the latest map you can get hold of. In the rural areas it’s not that much of a problem as the hills and natural features won’t change that fast but, it’s always good knowing the up to date rights of way and locations of private land. In the more urban areas things can change quite quickly. You might be planning a nice quiet walk through a field when in reality you’re now walking through a new housing estate because you’re map is 10 years old.
Grid Magnetic Angle
The grid lines on your map pointing north and the needle on your compass do not point to the same place on the planet. If you imagine a line running to each location it would create a V shape. The angle between these two lines is known as the ‘Grid Magnetic Angle’. Just to make things more complicated, magnetic north is constantly moving. Not by much but enough to have to make some changes.
Luckily for us, maps have this information printed to help us out. It’s always handy to work out the GMA before hand and write it somewhere on your map for future reference.
If you’re going from a grid bearing to a magnetic bearing then you’ll need to ‘add’ the GMA. Magnetic bearing to a grid bearing then you need to ‘get rid’ of the GMA. Only if the GMA is in the West, if it’s in the East then everything swaps over.
GMA West
Grid to Mag — Add
Mag to Grid — Get Rid
GMA EAST
Grid to Mag — Get Rid
Mag to Grid — Add
If I’ve lost you don’t worry as a separate post is coming real soon explaining this subject in greater detail.
Scale
The last thing to check is the scale of the map. Walking/hiking maps generally fall between the scales of 1:25,000 and 1:50,000. Anything less tends to be too large and you need to carry a lot of mapping for a long journey. Anything more and the map starts to lose information needed for a day out in the hills.
Ordnance Survey produce maps in both those scales, the 1:25,000 has a fantastic amount of detail. A nice happy balance would be the maps by Harvey, and they produce a 1:40,000 scale map which sits nicely between the two. Definitely something worth considering when choosing the scale of mapping for you journey.
A good all round navigator will have experience with a variety of scales as each have their pros and cons. Take note of the scale on the map you have as this makes a difference when calculating distances.
Thank you for taking the time to read this post. I am a qualified Land Navigation instructor in the British Army and I’m going to produce more content on this subject here on this blog. Please don’t hesitate to contact me with any questions you may have.
Aaron.
