SC120 Map Interpretation, Air Photos, and GPS

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SC120 Map Interpretation, Air Photos, and GPS

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Module 3 - Map Coordinate and Projection Systems: Universe Transverse Mercator

Reading Assignment: Chapter 6 of Sebert text.

Map Projections

Universal Transverse Mercator (UTM) is both a map projection and a coordinate (or grid) system.

As a projection system, UTM divides the earth into transverse (north-south) strips called UTM Zones.

UTM system is used in Canada's popular 1:50,000 and 1:250,000 scale topographic maps.

As a coordinate (or grid) system, UTM uses rectangular grid - a system for finding locations on maps.

UTM Map Projection

The earth is 360^o in circumference.

UTM projection system divides the earth into 60 vertical UTM zones, each 6^o wide (60 zones x 6^o = 160^o). (Imagine sectioning an orange peel into 60 equal vertical strips and flattening each strip).

Because the zones are very narrow and long, the distortion caused by map projection is minimal and can be ignored for practical purposes.

Each UTM zone extends to 84^oN Latitude and 80^oS Latitude.

These zones are numbered 1 to 60.

16 of these UTM zones (UTM Zone 7 to 22) cover Canada as shown below.

UTM zones can be divided into a basic set of map sheets of a convenient size.

These basic sheets are further divided into sections and each section is published as a map.

In Canada this is done to produce the 1:250,000 and 1:50,000 scale maps, which are the two scales of the National Topographic System (NTS).

By convention, the edges of the map fall along parallels and meridians.

In the far north, where zones are narrow, medium- and small-scale maps may show parts of more than one UTM zone.

Important feature of UTM -- Zones are unique (standard) and readily identifiable, so that an area may easily be identified as lying within a specific zone.

UTM Map Coordinate or Grid

UTM coordinate or grid uses UTM lines as its basic element of identification.

These lines are unique (standard) on all UTM maps.

Because the UTM lines are drawn on long, narrow UTM zones, the grid lines are perfectly straight and undistorted.

Basic grid line, both horizontal and vertical, are drawn 100,000 meters (100 km, or about 62 miles) apart.

All vertical lines run parallel to central meridian of each UTM zone (the meridian that runs down the cnter of each zone 3^o longitude from either side).

All horizontal lines run parallel to the equator.

Important Note:

Parallels of latitude shown on UTM maps are not parallel to the equator, or to each other, because UTM projection slightly distorts them, but does not distort the equator.
Every map projection produces some kind of distortion on maps because map projection is representation of surface features of a spherical earth on a flat map.

Depending on the scale and purpose of the map, squares formed by the intersection of the 100,000-meter lines are typically subdivided into:

10,000-meter lines
1,000-meter lines
100-meter lines

Most maps portray an area less than the width of a UTM zone, but they all provide information about the UTM zone and UTM grid spacing on the margin of the map, in the following way:
ONE THOUSAND METER
UNIVERSE TRANSVERSE MERCATOR,
GRID ZONE 12

By using a brief code consisting of zone and grid-line numbers it is possible to identify any point in Canada, even if that point is not otherwise marked on any map.

UTM maps use two UTM grid referencing systems:

Civilian UTM Grid Reference System
Military UTM Grid Reference System

Civilian UTM Grid Reference System

Horizontal lines are designated by their distance from equator in meters.

Because Canada's southernmost point is about 4,620,000 meters from the equator, all horizonal lines in Canada have a "northing" value above that figure.

Vertical lines are measured from a separate point for each zone, namely, an imaginary line lying 500,000 meters west of the zone's central meridian.

Even though 500,000 meters on either side of the central meridian suggests a total width of 1,000,000 meters, zones never attain the full width of 1,000,000 meters.
In northern Canada, zone widths shrink to as little as 80,000 meters (40,000 meters on either side of the central meridian).
Therefore, vertical lines are counted from the central meridian or 500,000 meter line.
Vertical lines to the left of the central meridian have an "easting" value of less than 500,000 meters and those on the right have a value above that.

The number of meters north of the equator represented by the bottom horizontal grid line on a map is always shown in light-blue color in the lower left-hand corner of the map.

If a given point on a map is positioned exactly at the intersection of a vertical and horizonal line, its location may be read off simply from the map margins.

Its full designation or its "coordinates" would include the zone number, followed by easting, and then northing values. On a 1,000-meter grid, these coordinates might read:
12,357000,5476000

On a map, the values of the first vertical and horizontal lines appearing on the bottom-left corner of the map are given in full (e.g., 357000m for easting or 5476000m for northing). The other grid lines are abbreviated fashion (e.g., 58 for easting or 77 for northing, both of which are 1000 meters to the east or the north of the previous easting and northing lines).

If the point does not fall at the grid intersection (which is usually the case), it is necessary to measure or estimate the distance to the nearest vertical line to the west and to the nearest horizontal line to the south and add these metric values to the grid values given at the margin.
Example:
If the church is located 400 meters east of the vertical line of 357000m, and 200 meters north of the horizontal line of 5476000m, its coordinate would be 12,357400,5476200. With these three numbers, any point on the northern hemisphere can be unmistakably identified.

Because civilian UTM grid coordinate system uses only numbers, it can be handled by Geographic Information Systems (GIS) and other digital mapping software -- an important consideration with any kind of digital data.

Civilian system does, however, require the use of large, somewhat cumbersome figures. To get around this military map-makers have developed a somewhat different system consisting of a combination of letter and numbers -- the Military UTM Grid Reference System.

Military UTM Grid Reference System

Topographic maps have rectangular UTM grid with grid reference numbers in the margin identifying the horizontal and vertical lines.

On large-scale maps (1:50,000 and larger) each number consists of two digits as shown below:

Question: How do you arrive at a map reference for the church shown in the figure?
Answer:

"Easting" is given first. Easting is given with respect to vertical line to the left of the church, which is 91. The church is eaast of vertical line 91.
"Northing" is given second. Northing is given with respect to horizontal line to the south of the church, which is 94. The church is north of horizontal line 94.
Therefore the designation of the square containing the church would be 9194.
To arrive at the reference for the church itself, following two methods can be used:

Rough Visual Estimation
We can imagine the square divided into 100 smaller squares (10 x 10).
Visually estimate by eye that the church is:

6/10th of the way between vertical grid lines 91 and 92.
4/10th of the way between horizontal grid lines 94 and 95.

Using these figures, the easting would be 916 and northing would be 944.
By convention these are combined into a reference of 916944.

Precise Estimation with Romer
Roamer is a small transparent card graduated with units of a proper scale shown in the figure. If the card is unavailable, you can improvise one by marking units on a piece of paper by placing the margin along a proper secondary bar scale.

With roamer, a more precise (than 916944) church location reference can be found.
A more precise church location could be 91559435.
The reference always has an even number of digits, the first half representing the easting, the second half the northing. It is not necessary to separate the easting and northing by a comma.

Because military UTM reference numbers repeat themselves every 100,000 meters (100 km or about 62 miles), 100,000-meter squares are identified by letters which are printed in blue on all NTS maps.
The diagram below shows a reference on a 1:250,000 scale map (where grid lines are identified by a single number).

In this example, the church would be in square NT99 and more precisely (determined visually or by using a roamer) at point NT9293.
This reference is sitll not unique, but does not occur again for about 2,900 km (1,800 miles). For most purposes, this is sufficiently unambiguous.
If a unique reference is required, Grid Zone Designation (found on all NTS maps) are added, such that the reference now would be 18 T NT9293
In this reference, 18 is the UTM zone, T is general area north or south of the equator in bands of 8 degrees

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