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Map Coordinate Systems #210
    Date first written: 2003.11.13   Review Date:2016.07.20

A discussion of coordinate systems is essential to reconcilling with most of the Canadian 1:50K paper maps. Paper maps were much more important before GPS maps came into widespread use.

Your position on the earth can be described in two different coordinate systems: Lat-Long and UTM. Don't confuse coordinate systems with "Map Datums". Both coordinate systems depend on which "map datum" you are using. (A lot of people are still confused about this point, and think that latitudes and longitudes are "absolute" and don't depend on the Map Datum. Unfortunately, no.

"Lat-Long" means latitude-Longitude. For example, 50 degrees and 29 minutes north, 123 degrees and 50 minutes west. Latitude-Longitude coordinates are sometimes referred to as the "geodetic grid" which we abbreviate to "Geo". "UTM" means "Universal Transverse Mercator". Canadian Topographic maps have both scales, but only have grid lines for the UTM scale. The Lat-Long system is simpler and more universal, and is what is always used behind the scenes in computer systems.

A set of coordinates alone does not specify your exact position, because you need to know what "map datum" applies to those coordinates. For example, if someone tells you that the US-Canada border is at 49 degrees north, you have to ask what datum they are using. 49 degrees by what datum? See Border Slash - Is it really the 49th parallel.

There is about a 200m difference, depending on the datum. The two main datums in Canada are WGS84 and NAD27. "NAD27" means "North American Datum 1927". "WGS 84" means "World Geodetic System 1984". Eventually all maps will be WGS84, because it is based on satelite measurements. However, most Canadian topographic maps are still NAD27. The difference can be as much as 200 meters, depending on your location. Here are some comparisons of the different co-ordinates for the same point: (Mt. Garibaldi)

 WGS84 Geo coordinates: 49.850-123.008
 NAD27 Geo coordinates: 49.850-123.0066
 WGS84 UTM coordinates: 10 U 499425 5521953
GPS receivers can be configured to display your true position in a variety of position formats and datums. For example, you can set your GPS to read out a given waypoint in WGS84 UTM coordinates. Then you can change the readout to NAD27 Canada, and you'll see different coordinates for the same point.

The Bivouac website has a similar capeability. You can display the waypoints in any one of several common combinations. The default is WGS84 Lat-Long. But if you want to mark those points onto a typical canadian topo map, then you should click the "UTM-NAD27C" link. This will redisplay the waypoints as UTM, and using the NAD 27 Canada datum.

Now that you know the general idea of lat-longs, UTM, and datums, we can now look at the different notations for writing these things down.

Lat Long Notations:
Ideally, latitudes and longitudes would be given as decimal numbers, like any other number. For example: 50.778 degrees North. But for historical reasons, most paper maps have their scales in degrees and minutes. A minute is 1/60 of a degree. So we have to accommodate that format and translate it to decimal behind the scenes. And many computer systems display in Degrees-minutes-Seconds.

Given that we have to input values in degree-minute or degree-minute-second format, the first problem is that there is no degree symbol on the keyboard. Although we can display lat-longs using degree symbols (Eg: 50 ° 27.2') you can't directly type such a thing, so we use a colon for the separator between degrees and minutes. For example:


Note that in the above, there is only one colon, and the fractional part of a minute is delimited by a decimal point. That format is generally referred to as "Degree-Minute" (DM) format. The other common format you see in a lot of digital mapping packages is Degree-Minute-Second format (DMS format). In this format, the previous lat long becomes:


Note that the .5 minutes becomes 30 seconds.

The next requirement is a standard for writing both coordinates. We always put the latitude first, followed by the longitude. This has generally been the standard for the past 3 centuries, although lately you will see systems like BC Basemap which have it reversed.

Given that the latitude will always be first, and that all the values we are dealing with are North and West, we don't need to put "N" and "W" into our coordinates. All of North America is north and west. Therefore we just write our lat-longs as two strings separated by a dash. Anywhere in Bivouac that you are supposed to input a lat-long, they are combined. Below is the position of Mount Garibaldi in DM format:


Bivouac will accept lat-longs in any of the three formats: decimal degree, degree-minute, and degree-minute-second format. Which format is used usually depends on the source of the data: anything coming from BC Basemap will be in DMS format, and most points from paper maps will be DM format.

Note that our format is only good for North America. Most worldwide computer systems use plus and minus to indicate North versus South, and east versus west. In this case, you would have to separate the lat and long with a space, as shown below:

49:51.0 -123:00.5

UTM Formats
Before computers, the traditional method used by Canadian mountaineers was to communicate positions by using friendly looking grid references like "234-234 on map 92 G/15. The reason it looks simple is because nobody bothered to specify the datum, and they also threw away the upper digits, because they were giving the mapsheet. However, such simple minded grid references don't work once you get away from paper maps. Nowdays, if you are going to use UTM coordinates, you should understand the full UTM system. (Or use Lat-long).

The full blown UTM format has four parts as shown below:

 10 U 499425 5521953

The first is the "Zone number", then the "Zone Letter", then the "Easting" and then the "Northing". Note that for some reason the UTM gives the east-west coordinate first, rather than following the lat-long standard of giving the north-south coordinate first.

The last decimal place is accurate to exactly 1 meter. Because a full UTM code is cumbersome, the grid lines on 1:50,000 maps are labelled by rounding off the last three digits, and also throwing away the first digits, such that only 2 digits remain. In the corner of the map the full number is given, but then each subsequent line is labelled with a two digit number, as shown below:

  64 465000m 66 67 68 69 70 71 72 73 74

The above grid lines are 1 km apart. In mountaineering guidebooks, grid references are usually written to the nearest 100m which is about as accurate as you can estimate from a map. For example, an article might state that the whole trip was on map 92G/6, and then give grid references like GR 878-569 or GR 878569.

Bivouac Co-ordinate Systems
All data is STORED in lat-long format, with WGS84 as the datum. However, in order to make it easy to use printouts of road pages or trip pages in the field, we went to a HUGE effort to write a conversion program that will allow you to display the data in a format that matches the map you are using. In most maps are UTM and NAD27, so a double conversion must be done. You can change the format of the waypoints to UTM and NAD27 by clicking the link labelled "UTM27". You will then see each of the waypoints in truncated UTM format, and NAD 27 Canada. If you happen to be using a newer map, click on "UTM84".