Map reading and field study

FOCUS QUESTIONS

What is a map scale? What are the different types of map scales? How to measure distance on a map?

Maps are a physical representation of the Earth.

The scale of a map shows how much you would have to enlarge your map to get the actual size of the piece of land you are looking at.

MAP SCALE

Is a ratio of the distance on a map to the actual distance of the ground. OR

Refers to the relationship (or ratio) between distance on a map and the corresponding distance on the ground.

On a 1:100,000 scale map, 1cm on the map equals 1km on the ground.

Whether the scale is drawn out or written, you can easily calculate distances if you know the ratio.

THE SCALE IS PRINTED AT THE BOTTOM AND TOP OF EACH MAP

The smaller the map scale, the larger the reference number and vice versa. For example, a 1:100,000 scale map is considered a larger scale than a 1:250,000 scale map.

Mapmakers use a mathematical ratio, called a scale, to shrink the distance and area of a place. The scale determines how much area can be shown on a map.

The scale can be expressed in the following two ways:

As an equation. An example is 1cm = 10 km. The equation means that 1 centimetre (cm) on the map represents 10km (kilometres) of actual earth surface.

As a ratio. An example is 1:50,000. The ratio means that 1 unit of measurement on the map represents 50,000 of the same units on the earth’s surface. When the units are not specified, as in this ratio, it is understood that the units are the same on both sides of the ratio. Let’s look deeper at types of map scale

A GRAPHIC SCALE/BAR SCALE

Depicts scale using a line, with separations marked by smaller intersecting lines, like a ruler.

One side of the scale represents the distance on the map.

The other side represents the true distances of objects in real life.

It is easy to calculate the actual distance between those same items.

It is a straightforward and easy way to determine scale.

If a map’s size is enlarged or reduced, the scale is also enlarged or reduced, meaning that it is still accurate.

A VERBAL SCALE

Uses simple words to describe a prominent surface feature. Uses simple words to describe the ratio between the map’s scale and the real world. Calculating scale on a map using verbal scale To do this, you must simply measure the distance on the map and then follow the verbal directions to calculate the actual distance. It is easy.

A FRACTIONAL SCALE

Shows the fraction of an object or land feature on the map.

Uses a set of numbers that represents the object or a landmark.

Uses representative fraction to describe the ratio between the map and the real world. This can be shown as 1:50,000 or 1/50,000. This mean that.

1 unit of distance on the map represents 50,000 of the same units of distance in the real world.

This means that 1 centimetre on the map represents 50,000 centimetres in the actual ground.

Representative fraction scale has the same unit of measurement on both sides of the colon.

It is easiest to calculate distance using a stated scale, but most maps don’t include stated scales.

TO CONVERT FROM REPRESENTATIVE FRACTION TO VERBAL SCALE

You convert the fraction to familiar units of measurements. For example: 1:250,000. 1 inch equals 250,000 inches (63,360 inches = 1 mile).

Verbal scale = 1-inch equals 4 miles.

HOW TO MEASURE DISTANCES ON A MAP

STEPS

1. Find the scale for the map you’re going to use – it might be a ruler bar scale or a written scale, in words or numbers.

2. Use a ruler to measure the distance between the two places. If the line is quite curved, use a string to determine the distance and then measure the string.

3. If the scale is a representative fraction (and looks like 1/100,000 or 1:100,000), multiply the distance of the ruler by the denominator, giving distance in the ruler units.

4. If the scale is a word statement (i.e., ‘One centimeter equals one kilometer’) then determine the distance.

5. For a graphic scale, you’ll need to measure the graphic and divide the scale into the measured units on the ruler.

6. Convert your units of measurement into the most convenient units for you (i.e., convert 63,360 inches to one mile). OR To determine the actual distance between two places on a map by using the following procedure: 1. Find the scale printed on the map. The scale on the map shown is 1:50,000. The unit of measurement is centimetres (cm). Centimetres are not practical for measuring long distances, so convert the scale to kilometres. We know that 100,000cm = 1 km, so 50,000cm x 1 km = the number of km for the scale. 100,000cm

1 cm = = .5km

Therefore, 1cm on the map = .5km on the ground. The scale is 1cm = .5km. 2. Using a ruler, measure the distance between two locations on the map and multiply that distance by the scale.

FOR EXAMPLE

Locate the two Xs on the map. Measure the distance between the two Xs. The distance is 4.82cm. Multiply 4.82cm by .5km and you’ll get = 2.41km.

The distance between the two locations is 2.41km.

CURVE DISTANCE

The distance between two locations is often not represented by a straight line.

You may need to measure irregular shapes on a map, such as twisting roads or coastline.

To measure the distance represented by an irregular shape, follow these two steps:

1. Find the scale on the map. If the unit of measurement is cm, it may need to be changed to km. Do you remember how to convert a scale from cm to km? How to convert a scale from cm. to km. Suppose the scale is 1:50,000. The scale does not specify units of measurement; therefore, we know that the units on both sides of the scale are the same. If the unit of measurement is centimetres (cm) then the scale is understood as 1 cm: 50,000 cm. Kilometre is a practical measurement for long distances so you will want to change cm to km.

Since 100,000cm. = 1km, 50,000cm x 1 km = .5km. The converted scale is 1cm=.5km.

100,000cm

2. Lay a piece of string on the path that you want to measure. Using a pen, mark the string where it touches the two locations between which you want to measure. Using a ruler, measure the distance between the marks on the string. Multiply that distance by the scale. If the measurement between the marks on the string is 6 cm. then the distance of the irregular path is 6 x .5 km = 3.0 km.

Take a ruler or the edge of a piece of paper and mark on it the straight-line distance between your two points.

b. Then, put the ruler or the paper just under one of the bar scales and read the ground distance in miles, metres or yards.

c. Suppose you want to find the distance between A and B around a curve in a road. Take a strip of paper, make a small tick mark on it, and line up the tick mark with point A. Align the paper with the road edge until you come to the curve, make another mark on the paper and on the map, and then pivot the paper so that it continues to follow the road edge. Keep repeating this until you get to B. Always follow the road edge with your paper. Make a mark on your paper where it hits B, and then go to the bar scales to get the distance. 2. Normally, you will be required to measure distance in metres, and you may receive a problem that goes off the bar scale. The metre bar scale allows you to measure distances up to 5,000 metres.

If you must measure distances greater than 5,000 metres, follow this procedure:

a. Place your starting point on the paper under the zero on the bar scale. Measure off 4,000 metres and place a new tick mark at the point on your paper.

b. Place this second tick mark also under the zero on the bar scale and determine if the distance on the paper now falls within the bar scale. If it does, add this value to 4,000 to give a total distance. If it does not, repeat the last step (a) until the distance on the paper falls within the bar scale. Remember to add this last value to the total number of metres you have already measured.

IN SUMMARY MEASURE DISTANCE ON A MAP

1. Measure the straight-line distance on the map using the straightedge.

2. Place the paper under the metre bar scale.

3. Determine the distance with no more than 5% error.

4. Measure the curved-line distance using the strip of paper.

5. Place the paper under the metre bar scale.

6. Determine the distance with no more than 10% error.

Next class will give direction in terms of compass bearing and the 16 points of the compass.