Difference between revisions of "Navigation"
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This page is all about wilderness navigation for hiking, mountaineering and ski touring.
This page is all about wilderness navigation for hiking, mountaineering and ski touring.
Latest revision as of 23:38, 26 November 2014
This page is all about wilderness navigation for hiking, mountaineering and ski touring.
- 1 Maps
- 1.1 NTS 1:50,000 map series
- 1.2 TRIM 1:20,000 map series
- 1.3 Coordinates
- 1.4 Three different kinds of North
- 1.5 Datum
- 1.6 Reading Topographic Maps
- 2 The Compass
- 3 GPS
- 4 References
For information about free online electronic maps, see the Online mapping page.
NTS 1:50,000 map series
Most recreationists in Canada use the National Topographic System (NTS) 1:50,000 mapsheet series. These maps are convenient scale that covers a large area but also shows sufficient detail.
Each NTS map has a 3 part code. For example: 92 J/3 is the map for Brandywine Falls, the Brew Hut and Rainbow Mountain. The whole country is divided into a grid of 1:1,000,000 scale maps. Southwest BC is map 92 (9 = map index from east to west, 2=map index from south to north). All smaller maps that fit within map 92 are prefixed with the code 92. Each 1:1,000,000 map is divided up into 16 1:250,000 mapsheet that are index with a letter from A-P in a back and forth fashion. Then each 1:250,000 map is divided up into 16 1:50,000 mapsheets that is index with a number between 1 and 16, again in a back and forth fashion. All this is very confusing, so the government has created index maps that show the relationships of the the smaller maps to the larger ones.
NTS maps come in 3 different forms. Usually each map is only available in one form, but sometimes you have a choice
- Tyvek maps are the best. They are printed on Tyvek which is waterproof and highly tear resistant. The only down side is it is very hard to write on them with a pen or pencil. Tyvek maps are a relatively new addition to the NTS map series. The number of maps available in Tyvek seems to be increasing, which is a good thing.
- Printed maps are pretty good. The image quality is the same as the Tyvek map series but the paper is less durable. These maps have been available since the beginning of time.
- Plotted maps are the worst. These maps have been printed on an inkjet printer by a 3rd party map printer and it shows. Image quality is poor and the paper is the same as on the old printed maps. Don't buy these maps if you don't have to.
NTS map contour intervals are 100 feet on the old maps, and 40 meters on the new maps. Bold contours are every 500 feet or 200 meters. The metric maps use the NAD83 Datum (same as WGS84) whereas the maps with contours in feet use the older NAD27 Datum. All maps are marked with Lat/Long around the edges and a 1km UTM grid across the entire map.
NTS mapping information and map images are freely available online. See the Online mapping page for details.
TRIM 1:20,000 map series
The BC Government has it's own set of maps that are known as TRIM maps. These maps use the same indexing scheme for the 1:1,000,000 and 1:250,000 levels as the NTS maps. However, the TRIM series uses 100 1:20,000 mapsheets to cover a 1:250,000 map. TRIM maps alway have 3 digits in the 3rd part of the code so that the codes are alway distinct from NTS maps. For example 92 J/1 is a 1:50,000 NTS map and 92 J/001 is a 1:20,000 TRIM map.
TRIM map contour intervals are 20m, and bold contour intervals are 100m. All maps use the NAD83 datum (which is the same as WGS84)
The data for TRIM maps is freely available online. See the Online mapping page for details.
Latitude and Longitude
The earth is essentially a sphere, so it makes sense to measure your position in terms of angles. Latitude is your north-south position. The equator is 0 degrees, the north pole is 90 degrees north and everything else is somewhere in between. Each degree is divided into 60 minutes, and each minute into 60 seconds (written like this: 49 30'45"N). For convenience, a nautical mile is one minute of latitude.
Longitude is your east-west position, measured from some old observatory in Greenwich, England. Longitude is demarkated in the same way as latitude using degrees, minutes and seconds. The distance between two lines of Longitude varies with Latitude, getting smaller and smaller as you move towards the poles. This makes calculating distances between two points based on their Lat/Long coordinates troublesome.
It is always necesary to specify E or W with longitude and N or S with latitude, although in all of North America it is always N and W, so sometimes the N and W are dropped.
Most coordinates on maps use Universal Transverse Mercator (UTM) coordinates (an invention of the US military). The Transverse Mercator is one of many map projections that transforms the roundish earth to a flat surface for printing or staring at on a computer screen. The UTM system standardizes the Transverse Mercator map projection by divided the earth into 60 six degree wide numbered zones for the northern and southern hemisphere. Therefore the map should state which zone it is in. Vancouver is in UTM Zone 10N (the 10th zone from the International Date Line).
Coordinates on the map are given as eastings (x) and northings (y). The UTM easting is roughly the distance from the zone’s central line plus an offset of 500 000 m to keep all the numbers positive. Since the coorinates are given in metres on a flat plane, it is generally easy to calculate the distance and direction between two points. In UTM Zone 10, the central line is at -123 degrees longitude. The northing is roughly the distance along the surface of the Earth from the equator. An offset is also added to the northing for zones in the southern hemisphere. It is important not to forget to specify which zone a UTM coordinate refers to for complete accuracy.
Truncated UTM Coordinated
Often guide books (103 Hikes in SW BC) will only give truncated UTM coordinates, instead of the full coordinates to the nearest meter. These may be 6 or 8 digits, with the first half referring to the easting (x) and the last half referring to the northing (y).
For example consider the coordinate reference GR946718. 6 digital Grid references are always given to the nearest 100 m, you can expand the x coordinate to 94 600. If your map has x values running from 464 000 to 500 000 then you know the full x coordinate is 494 600m. Similarly the y coordinate can be expanded to 71 800 and if the maps y values range from 5 456 000 to 5 483 000 you know that the full y coordinate is 5471800m. NTS maps helpfully truncate the coordinates printed on their map grid, so the first two numbers of the truncated UTM x and y coordinates allow you to easily find the 1 km grid you are interested in and the third number is an estimate to the nearest 100m within that grid.
Note that truncated grid references are not unique, as a particular grid reference is repeated every 100km N,S,E and W, so you must know approximately where the grid reference refers to. Knowing which NTS mapsheet you are on is sufficient. For this reason, truncated grid references are convenient when you want to look up a particular location on a map, but troublesome if you want to enter them into you GPS, since to do so you must reconstruct the full UTM coordinate.
Three different kinds of North
Maps show three different north arrows and the relationship between them.
- True north is the direction to the north pole, and the edges of NTS and TRIM maps are aligned to true north.
- Magnetic north is the direction you compass will point. The relationship between magnetic and true north is called "Magnetic Declination" and it varies with your location and also changing with time as the magnetic north pole of the earth drifts around. NTS maps will indicate the magnetic declination at the time the map was published and also the rate at which the declination is changing per year.
- Grid north is the direction of the UTM grid (the blue grid the is overlaid on all NTS maps). The relationship between true north and grid north varies with your location.
There is no such thing as an absolute location, it is only possible to measure location with respect to some reference point. Latitude and longitude have no meaning if you do not know what the reference point is. A datum is this reference point and maps are constructed by surveying from the reference point outwards.
A datum consists of a description of the shape of the Earth (usually an ellipsoid with a semi-major and semi-minor axis) and information about how this shape is tied to the Earth. Therefore, with a datum, latitude and longitude have a meaning in 3D space (on the surface of the ellipsoid) that can be then located on the Earth's surface.
Prior to satellite navigation, datums were only fitted to local portions of the Earth (North America for example). Each different part of the Earth used a different datum, and coordinates measured on one datum were not compatible with coordinates measured on other datums. Modern datums (NAD 83, WGS 84) are Earth centred and can be used for measuring coordinates on the entire Earth.
Datums typically encountered in North America are:
- WGS84 is the datum that the GPS system uses
- NAD83 (North American Datum 1983) is essentially the same as WGS84
- NAD27 (North American Datum 1927) is a local datum that was only used for North America. The reference point is Meades Ranch in Kansas. The difference between NAD27 and NAD83/WGS84 is about 200m off in the northing (y) and about 100m off in the easting (x). There is also a difference in elevation between WGS84 and NAD83 but it is only a few meters.
For general backcountry travel, it is only important to remember that there is a slight difference between NAD 27 and NAD 83. If you have an old map that uses NAD 27, the coordinates will not match what your GPS displays. You will either have to convert the coordinates to NAD 83, or ask you GPS to display coordinates in the older datum.
Reading Topographic Maps
Contour lines on a map are used to represent places that have the same elevation. Generally a topographic map will state its contour interval (the elevation difference between adjacent contour lines) and will label the exact elevation on some contours. Some maps also print a darker line for some contour intervals, so a map with a contour interval of 20m may print a darker line for the 100m contour lines.
If your route travels along a contour line, the route is essentially level. If you cross contour lines you will be traveling either uphill or downhill. The spacing between adjacent contour lines indicates the slope of the hill: lots of lines close together (or missing) indicate a steep slope, while few contour lines spaced far apart indicates a very low slope. You can calculate the percentage slope by counting the number of contour lines, multiplying this number by the contour interval to get the change in elevation (vertical distance) and then dividing by the horizontal distance (and finally multiplying by 100% to convert the number into a percent).
The pattern of contour lines, combined with the location of rivers and lakes, on a map can tell you what the terrain features look like as shown in the following images (pictures stolen from Touch, Wilderness Navigation Handbook).
Summit or Hill
|Similar to a valley, a gully is a groove running down a slope. From within the gully there will be high ground on three sides.|
|A vertical feature. The contour lines might be so close together that they will be touching or missing.|
Saddle or Pass
|A dip or low point between two areas of higher ground.|
Fundamentally a compass does one thing – it points North. Once you know what direction North is, you know all the rest of the directions by default. The difference between north and your direction of interest is called your bearing – it’s usually measured in degrees (360 all the way around), and sometimes degrees are divided further into minutes (60’ per degree). Your compass will allow you to ‘save’ a bearing by rotating a dial (containing the meridian lines and the orientation arrow – these line up with north) set within the compass housing relative to the base plate (which you will point in your target direction). There are lots of things you can do with this knowledge, but most require only four fundamental skills (in addition to your own cunning), outlined on the next page. So, I’ve already lied (sort of) – a compass needle does not point North. It points towards Magnetic North. Unfortunately Magnetic North and True North (the geographic North Pole, and usually ‘up’ on maps) are not the same place. Magnetic Declination is the local difference between True North and Magnetic North. Unfortunately you can’t solve this problem by printing a map with Magnetic North at the top, as Magnetic North changes slowly over time (over the course of history the earth has even flipped polarity entirely, but don’t fear – this happens of a timescale of hundreds of thousands of years).
Dealing with Declination
(If you’ve never used a compass before I highly suggest you scan this section, then read the section on the four basic skills, then re-read this section. If it doesn’t make sense on the first scan do not fear.)
If you don’t want to deal with Magnetic Declination you should move to Dryden, Ontario where it is negligible. If you live, say, in Vancouver then, as of August 6, 2009, the local declination is 17˚ 38’ East (17 degrees 38 minutes) – this is a lot, and will put you way off course if you don’t deal with it. For up-to-date declinations in Canada try this website: http://geomag.nrcan.gc.ca/apps/mdcal-eng.php. To add further complexity maps often have their Grid North somewhat different than True North as well. This information will be printed on the map. A declination of 17˚38’ E means that a compass needle will point to the east (right) of True North (as opposed to the other way around). There are a few ways of dealing with this, in order of complexity (obviously only one these methods should be used at a time…):
- Own a compass with a declination dial. Simply follow the instructions included with your compass (this usually involves turning a screw on the back) to shift the declination adjustment before you head out. This will allow the meridian lines on your compass to point in a different direction than the orientation arrow and makes declination adjustments automatic. This is a good solution, but some are emotionally attached to their old compasses, which are not fancy enough to have a declination dial.
- Draw lines on your map oriented towards Magnetic North before heading out on your trip. Simple, may get you laughed at, and definitely makes your map worthless for future trips when the declination will have changed. However sometimes when making maps you have to draw lines on it anyway, so you might want them to be oriented with the current Magnetic North. Use trigonometry and a ruler to get the angle right – it’ll give you a better angle than a protractor. For the non-mathematicians, tan(17˚38’) is about 0.318, so make a line which goes 10 cm up for every 3.18 cm to the right.
- When transferring your compass between the map and the real world (as discussed in the next section) turn the dial the correct number of degrees, in the correct direction, to correct for declination. You may have to think carefully the first couple of times you do it. Near Vancouver this means you should turn the dial 17.6˚ to the right when moving from map-to-reality, and to the left when moving from reality-to-map. It does take thought to do this correctly, and you lose some accuracy in that you must rotate the dial. If done correctly however, it may make you appear to be a hardman™.
Taking/Following a bearing in real life
Hold the compass level in front of you at arms length, pointed away from your body. If you have a sighting mirror fold it back so you can see the compass dial in the reflection and look down the sighting notch in the top. Turn your whole body, not just the compass in your hand.
- If taking a bearing turn your whole body until the compass points at the target, then rotate the compass housing until the orientation arrow lines up with the magnetic needle (take care that you don’t line up with the south end of the needle – and easy mistake to make) – your compass dial is now set in the direction of your target.
- If following a bearing rotate your whole body until the magnetic north arrow lines up with the orientation arrow
- you are now facing your target direction.
Taking/Following a bearing on a map
Lay the compass down on the map, and start ignoring the magnetic needle. Note that if your map uses a grid north that is different from true north (this usually seems to be the case in SW BC) then you may want to set the magnetic declination on your compass to the difference between grid north and magnetic north (rather than the difference between true north and magnetic north). The difference between true north and grid north is usually quite small however, so this minor adjustment can often be ignored.
- If taking a bearing from the map rotate the compass so that it points in your intended direction of travel (route selection is a whole different skill, we’ll assume you have it for now). Using the edge is easier than using the middle of your compass. Now rotate the compass housing until the meridian lines line up with the north-south grid lines on your map (take care that you don’t end up with north and south switched) – your compass dial is now set in the direction of your target.
- If transferring a bearing onto the map rotate the entire compass until the meridian lines line up with the north-south grid lines on your map – the compass now indicates the target direction on the map.
Traveling in a straight line
Presumably if you need a compass, you can no longer see your destination. Hopefully however, you have the correct bearing on your compass – either because you took it earlier when you could actually see your destination (before the whiteout, bushwhacking, darkness etc.), or because you have a map and know where you are currently and took your bearing from the map. This means you must now follow this direction, and not get off-course, or your bearing will no longer be useful.
- With reasonable visibility this doesn’t mean that you must follow a ‘straight line’ – often it is easier to find a prominent object which lies along your direction of travel, travel to it by the easiest route possible, then find a new object.
- If you are traveling in whiteout/darkness or without objects to sight off of travel becomes more difficult – you must constantly check the compass or you will drift off course. Local terrain will influence you as well – you may find that you drift downhill or towards easier footing. Your own footprints or a drawn out party may help you notice if you are curving slowly in one direction. Another strategy following a compass bearing in a whiteout is to put the navigator with the compass at the back of the group. From the back, it's easier to see if the group is correctly following the course. Instructions can be shouted out from the back up to the person in the front. If caught in these conditions decide carefully if you should continue, turn back, or bivy.
- A ‘back bearing’, (line up the south end of the needle) lets you double check to see if you went the right way.
Sometimes you want to know where you are on a map – and with some creativity this is possible. Simply restrict your possible locations, and find the intersection of these possibilities. Good restrictions can come from:
- A shoreline or terrain feature – perhaps you know which river you are on, or maybe which ridge.
- A bearing taken off a prominent feature – locate a prominent feature and take a bearing from it, now you know what direction it is from your current location to that feature. You can draw a line back from that feature on a map and know you must lie somewhere along that line.
- A contour line - if have a calibrated altimeter and know your current elevation.
- The direction of a feature – note the orientation of the river, ridge or the aspect of the slope you are currently on, and look for similar places on the map.
- Any metal objects you may be carrying, and sometimes even iron ore in the rocks, can offset your compass needle – recheck your bearings frequently for consistency.
- Declination on old maps is no longer useful – investigate the current local declination before you leave.
Some More Compass Topics
When traveling along a bearing, you might not know when you will arrive at your location, since you likely will not be keeping track of your exact distance traveled. Therefore you should steer towards a catching feature, such as a lake, river, road, etc that you will be unlikely to miss. When you arrive at your catching feature, you should then know where you are on the map.
Often when trying to find a point feature (your car, a hut) along a linear feature (a road, lake shore) you may not know which way to travel once you intersect the linear feature. The car could be to your right or left. In these situations, you can introduce a deliberate offset to your compass bearing. If you add 5 or 10 degrees to your bearing, then when you hit the road you know that the car should be to your left down the road.
Besides using the compass to sight to something, you can also use it to sight backwards to an object. This can be useful if you want to check that you traveled in a straight line (sighting back to the position you were at or along your footprints in the snow). It can also be used to get around an obstacle: sight to an object on your direction of travel (i.e. a tree), move around the obstacle on your path and then take a back bearing to the object from the other side of the obstacle. Once you have moved to a position where the back bearing lines up with the object, then you are back on your original path. On most common orienteering compasses, a back bearing is achieved by lining up the red end of the compass needle with the black end of the orienting arrow (180 degrees off of normal). Be careful, since lining up the north end of the magnetic needle with the south end of the orienting arrow is a common mistake when taking forward bearings.
A Global Positioning System (GPS) is a handy device that uses the distance from three or more satellites of known location to calculate your position on the Earth. In mountainous terrain, 4 satellites are needed to also establish your elevation, otherwise large horizontal error will occur. The functions of a cheap consumer GPS generally include:
- calculating and displaying your location to +-10m (usually UTM coordinates using a WGS84 datum)
- saving your location as a waypoint
- entering new waypoints
- calculating the distance and bearing from your current location to a waypoint
- navigating to a saved waypoint
- showing which satellites are available and their position in the sky (used to generate an estimate of the accuracy of calculated coordinates)
More advanced units will:
- display a digital map
- have a magnetic compass
- have a barometric altimeter
With a GPS it is generally possible to find locations (ex. huts) in bad weather and whiteouts (when surrouding references needed for compass navigation cannot be seen). Map and compass skills are still important for situations when the GPS decides to go on strike. GPS's tend not to work well when dense forest or mountains are between you and the GPS satellites. The elevation calculated by a GPS also tends not to be very accurate unless many satellites are available.
While a GPS will tell you where you are in a total whiteout, it does not know the locations of hazards like crevasses or avalanche slopes, which may be difficult to see until it's too late.
Free GPS topo maps can be found here: http://www.ibycus.com/ibycustopo/. In linked youtube videos one can find instructions on how to download the maps and how to download the Garmin software that can be used to transfer maps to the GPS. Instructions are for Windows users.
If you need topo maps for the US (eg Washington), you could install the Northwest Topo maps:Northwest Topo maps. They are also fee and work with Garmin/mapsource. At the link above you can find the download file and instructions on how to download them.