Introduction to Map Use

study outline – basic mapping process

You should be able to:

1. describe the different models of the shape of the earth and give it’s basic dimensions

2. define latitude and longitude

3. give the latitude and longitude of a location on a map or globe

4. find a location on a map or globe given it’s latitude and longitude

5. describe the role of a datum in surveying

A. size and shape of Earth

1. Earth is basically a sphere – about 24,000 mi. (40,000 km) in diameter and 8,000 mi (13,000 km) across

2. More accurately, Earth bulges a bit at the equator so it is an ellipsoid. It is 27 miles thicker at the equator than pole to pole.

3. More accurately still, Earth’s mass is not quite evenly distributed so it bulges and sinks away from a perfect ellipsoid +/- about 100 yds in a shape called a geoid

B. latitude and longitude – a reference system for determining location on Earth

1. latitude – the angle between a plane through the earth a the equator and a line perpendicular to Earth’s surface (0-90 deg N and S)

i. can be measured by the angle of the north star (or other stars) above the horizon

ii. latitude lines are called parallels

iii. a deg of latitude is about the same length anywhere on earth

iv. a minute of latitude is a nautical mile

v. careful measurements must take slight variations in vertical due to gravity changes and the equatorial bulge of the ellipsoid into account

2. longitude – the angle between a plane through the arbitrarily designated prime meridian and a plane through the location in question and the poles

i. can be measured by noting the time difference between local noon at the prime meridian and the location in question (1 hr = 15 deg)

ii. lines of latitude are called meridians

iii. careful measurement requires taking small variations in the length of a day and the timing of noon into account

3. parallels and meridians meet at right angles

C. surveying is the process of measuring locations on Earth .

1. There are two basic types of surveying

i. geodetic surveys make a precise determination of location taking the curvature of the earth into account

ii. plane surveys make local measurements of position assuming the Earth is flat

2. surveying requires a control network of points with known latitude and longitude to serve as a starting point

i. a set of agreed upon control points is called a datum. The datum for horizontal and vertical measurements can be considered separately

ii. improvements in surveying and navigation have lead to several changes in the horizontal North America Datum

(1) some location have changed their official position by 10s to 100s of meters in different data

(2) when making precision measurements or doing precise navigation you need to know the datum that was used to produce the map or position coordinates

iii. measurements of elevation must refer to a vertical datum

(1) the international standard of vertical datum is sea level

(2) precise determination of sea level is also difficult and has had several agreed upon standards