A PARTIAL MAPPING GLOSSARY (DRAFT)
This is not a section on how to use maps (or a GPS, or compass). It is to sort out some of the arcana of mapping when you make your
own maps from online sources, or try to integrate maps from various sources. For example, you might want to compare a Parks’ map,
with Google Earth, and with an NTS map (try Skaha Bluffs). The Nordic XC Area map is another charmer. In fact this is a
compilation of all the bits and pieces I put together to try to rectify these maps. (I’ve failed so far.)
DO NOT RELY ON ANY OF THIS INFORMATION FOR NAVIGATION.
If you use a GPS with paper maps, you need to know what datum your maps are using. For NTS maps this is NAD 83 or NAD 27.
You will also need to know the zone (10 or 11, for the book). For BC government maps the datum is BC Albers. This is marked on the
map. You must match your GPS to the map grid, or you may get into trouble. (WGS is close enough to NAD 83, but for NAD 27, or
anything else, you need to change it.) You also need to note that real north is not quite true north, which in turn is nowhere near
magnetic north. If you have any other map you need to know the datum and projection. For BC government maps the protection is
usually BC Albers. For thematic maps (road maps) it’s usually Mercator. WGS 84 is usually a safe datum to use. For Google Earth
and Google Maps, you need a Mercator projection with a spherical datum.
Welcome to the bottomless black pit of mapping.
Albers (BC): The BC Albers Equal Area Conic projection is used for mapping by the BC government. It preserves areas better
than other projections, and the grid lines are like UTM, but the numbers are different, the grid is tilted, but there is only one zone
(instead of 5). This datum preserves area and distance well (better than UTM). This makes it important for resource allocation.
An NTS map, for the Kelowna Area, with a (black) UTM (NAD 83) grid overlaid,
and then rectified to BC Albers (orange grid). The arrow is true north.
The projection for Albers Equal Area Conic (BC Albers), has parameters of :
Central meridian:
-126.0 (126:00:00 West longitude)
First standard parallel:
50.0 (50:00:00 North latitude)
Second standard parallel:
58.5 (58:30:00 North latitude)
Latitude of projection origin:
45.0 (45:00:00 North latitude)
False northing:
0.0
False easting:
1000000.0 (one million metres)
The datum is NAD83, based on the GRS80 ellipsoid. Note the minus sign. Western longitude’s are negative by convention.
datum: A datum describes the model that is used to match the location of features on the ground, to coordinates and locations on a
map. NTS maps use NAD 83 (North American Datum), but older ones use NAD 27.
geoid: The Earth is not a sphere, it bulges at the equator, and is flattened at the pole, it's approximately an oblate spheroid. We
need to correct for the this, or rather define a reference ellipsoid; this is very complex to do. The universally (Earthily ? - Mars has its
own ellipsoid) accepted spheroid is the one defined in WGS 84 (which replaced the almost identical GRS 80 ellipsoid you may see
occasionally mentioned). One projection uses a perfect sphere.
geographic coordinates: A map position given in latitude and longitude.
latitude: The latitude of a point on the Earth's surface is the angle between the equatorial plane and the straight line that passes
through that point, and through (or close to) the center of the Earth. Lines that join points of the same latitude, trace circles on the
surface of the Earth. These are called parallels, as they are parallel to the equator and one another. These are the horizontal lines on a
map (if it is using a geographic grid). The 49th parallel is a latitude. The popular local wine, Latitude 50 is made at the 50th latitude,
i.e., 50o north of the equator. One second of latitude/longitude is about 30 m (100 ft).
longitude:
The longitude of a point on the Earth's surface is the angle east or west from a reference meridian* (usually at
Greenwich) to another meridian that passes through that point. All meridians converge at the north and south poles. *A meridian is an
imaginary arc on the Earth's surface from the North Pole to the South Pole.
Mercator Projection. This is made by wrapping a cylinder of paper around the earth with its long axis parallel to the Earth's
axis (see projection). The features are distorted, northern countries will look bigger than they are (relative to the equatorial countries).
It is difficult to transfer readings to and from a map. It is used mainly for marine navigation and thematic maps (e.g. road maps and
tourist brochures). These maps use a geographic grid. The faint curved grid lines on an NTS map are the latitude – longitude lines.
The Mercator projection. Note the area and shape distortion of the extreme latitudes. Details of the grid are shown below.
Mercator grid (orange) overlaid on an NTS (UTM, NAD 83) map of the Kelowna Area. Bothe UTM and Mercator grid are
locally rectangular, but as noted, the Mercator projection seriously distorts northern areas at large scales (Greenland
looks like the size of Africa, even though it’s 1/10th the size). Converting latitude/longitude coordinates to nothings/eastings
in the field is no fun. (Excuse the label mess, but my software doesn’t give me much control.) The arrow is true north.
NAD: North American Datum. Roughly speaking, it is a map grid that allows for the fact the earth is not a sphere. See WGS 84,
which now supersedes this datum. There are two variants; NAD 83, and the older NAD 27. NTS maps use NAD 83, but older ones use
NAD 27. The difference between NAD 83 and WGS 84 is insignificant in North America, but the deviations between NAD 83 and
NAD 27 are significant.
A section of the NTS map for the Twin Lakes area. The only available NTS map is NAD 27 (pale blue grid). The
NAD 83 (WGS 84) grid is black. You could probably find the lakes us NAD 83 coordinates with the NTS map.
Finding the parking lot (not shown) if you were bushwhacking would be a problem, 200 m in bush is a long way.
North: The book maps have grid north at the top of the page – they use the UTM grid. Magnetic North is about 19° east of True
North (and decreasing). UTM Grid North is about 2° west of True North.
NTS: National Topographic Series (of maps) produced by the government of Canada: incorrectly referred to as GSC maps by oldtimers. See above for a section of one.
projection: Imagine a light bulb at the center of the Earth, and the Earth is translucent, so it glows when the light is on. Now, if
we hold a flat piece of photo paper near to it the features on the Earth are projected onto the paper. That (when developed) will be
your map (in a planar projection). Obviously there are problems, the features are distorted and that will depend on how you hold the
paper. There are standard ways of partly getting round this. You can project onto cylinders, cones or spheres. Each type of projection
has different advantages, disadvantages. Only four of projections are defined here: Albers, UTM, Mercator, Web-Mercator, and UTM.
For more information on projections see the pdf files located on this file’s calling page.
Some standard simple projections.
The term "normal cylindrical projection" is used to refer to any projection in which meridians are mapped to
equally spaced vertical lines and circles of latitude (parallels) are mapped to horizontal lines. The mapping of
meridians to vertical lines can be visualized by imagining a cylinder whose axis coincides with the Earth's axis of
rotation. This cylinder is wrapped around the Earth, projected onto, and then unrolled.
rectification: Image rectification is a transformation process used to projected an image onto a defined image plane. In this
context that means taking a map and transforming it to give it a projection and datum. This can be difficult to do as thematic maps are
often have a poorly defined projection, and a tilted perspective.
UTM (Universal Transverse Mercator) Projection: This projection is also made by wrapping a cylinder around the
earth, but with its long axis in the plane of the equator see projection). Of course, you can rotate the cylinder around the Earth's axis,
so there are universal agreed set of places where the cylinder touches the equator. These are called zones. There are 60 zones of equal
width. The book area is mainly Zone 11U (120o-114o W), with some zone 10 (126o-120o W) hikes to the east. It’s based on a flat
trapezoidal (nearly rectangular on a 1:50000 NTS map sheet) grid that makes it easy to transfer coordinate data to and from the map. It
looks pretty much like what you would see in an aerial photograph. This grid has units of metres, and increases going north and east.
Eastings increase to the east (right) and are typically around 1440000 in this area; northings around 5500000 in this area. See NAD for
an example. Note the grid axis is slightly tilted away from true north.
WGS 84: This is a standard system for mapping and comprises a standard coordinate system (latitude, longitude), a standard
spheroidal reference surface (see geoid), and a gravitational equipotential surface (the geoid - the Earth is not uniform inside) that
defines the nominal sea level (where zero elevation is). This is the native system for GPSs.
WEB Mercator: This is used by Google Maps, Google Earth, and other similar on-line map programs. To simplify, it uses a
standard Mercator projection with a perfect sphere for the datum. This does not conform to WGS 84, and causes all sorts of problems
when one tries to match an NTS map, or even a Mercator map, to a Google Earth photo or Google (or Bing, or….). It is faster and
easier to calculate the coordinates this way (and thus uses less power - and given that Google uses about 1% of the world’s power,
that's not a trivial consideration). The US Government has banned the use of this projection in government documents. So rectify it
before trying to use it with any other map.