Map scales and generalization concepts

Map scale

- is the relationship between distance on the map to the same distance on the ground -usually found on the printed map or metadata Map scale= map distance/ ground distance -determines how much information is shown in a map and can be expressed in 3 ways (RF, verbal, scale bar)

Large scale

-Larger ratio -Smaller denominator -Smaller area -smaller map- usually St maps or ciites -More detail -1/500,000

Representative fraction

-RF -ratio or fraction of distance on the map to distance on the ground - one unit on the map represents x number of the same units in the ground - 1:24,000 = one inch on map represents 24,000 inches on the ground.

small scale

-Smaller ratio -Larger denominator -Larger area- big map- world maps, countries -Less detail -1/50,000,000

Elements on a topographic map

-author-agency on top right or lower right corner -quadrangle name and location -publication date -adjoining maps - location of the quad within a state -scale RF and scale bar formats -contour interval and vertical datum -north arrow and declination -longitude tick marks -bounding latitude and longitude values -UTM coordinates -Rd and trail symbols classification -map production info

Scale bar

-way to express map scale using what looks like a small ruler printed on the map -Distance can be converted easily from map to ground measurements by comparing map distance to the length of the scale bar

map scales

1. Representative fraction RF 2. Word statement or verbal scale 3. Scale bar (graphic symbol)

Generalization techniques

1. Selection 2. Simplification and smoothing 3 combination (aggregation)] 4. Locational shift and exaggeration 5. Conversions 6. Data classification

earths surfaces

1. earth surface 2. Ellipsoids-used to determine horizontal positions 3. Geoid-important to determine vertical measurements

Types of north

1.True north (geographical) 2. Grid 3. Magnetic north

ellipsoid

=sphere -is a simple apprx. To the earths shape -used for horizontal reference for locations, not use to measure elevations -semi minor (polar) 6,356,752.3m -semi-major (equatorial) 6,378,137m

Grid declination

Angular distance between the true north and grid north

Magnetic declination

Angular distance between true north and magnetic north

Conversions

Area features may be represented as points or lines -a river is represented as a line with equal width, city may become a point

geoid

Better model of the earths shape, not the same as the earths surface -used as a reference for elevation and vertical measurements -is complex and irregular -seperation between geoid and ellipsoid averages - 30 m

Advantage of scale bar over RF

If the original map is enlarged or reduced ex photocopy, the scale bar will be also valid on the resized map but neither original word statement or RF scale will be valid.

Medium scale

Is in between large and small -Europe (metric system)

Purpose of map generalization

Is influenced by the purpose for which the map is designed -two maps of the same area and similar scale can show different content depending on their purpose ex highway map vs topographic map -a map that did not generalize is useless but the value of the map depends on how it is generalized

Magnetic north

Marked by the needle on a magnetic compass -right of the true north

True north

North pole close to north star-Polaris. Meridians as reference line

Map generalization

Process of simplifying or generalizing a map to some extent by abstraction and removing detail or may be added-enhancements from the objects & phenomena to be mapped

selection

Retention of more important features and elimination of "less important" ones - smaller features may not be shown on map

geodesy

Science of measuring and monitoring the size and shape of the earth and the location of points on its surface

Grid north

The northerly direction indicated by the datum of the grid superimposed on the map. -to the left of the true north

Combination

Two or more similar features are combined using the same symbol (several forest fragments into one)

Locational shift and exaggeration

When showing overlapping features is not legible. displacement -change fonts, sizes etc

Word statement

a way to express map scale in commonly used map and ground units using descriptive expression. - "x inches to 1 mile" or " x centimeters to 1 kilometer" -map and ground units are usually different -do not use to calculate ground measurements - 1:24,000 = 1 inch to 2000 Ft - 1: 100,000 = 1 cm to 1 km

Simplification and smoothing

selectively reducing the shapes and features shown on the map (not all details is shown) -reducing the number of points to represent and object -reducing angularity of angles between lines -ex shorelines becomes less