Lecture #3 Maps and Map Scales

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Tracy Fisher
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1 Lecture #3 Maps and Map Scales

2 What is Map A representation, normally to scale and on a flat medium, of a selection of material or abstract features on, or in relation to, the surface of the earth map in mathematics is used to convey the notion of transferring information from one form to another

3 Topographic Map Types of Maps a reference tool showing the outlines of selected natural and man-made features of earth topography refers to the shape of the surface, represented by contours and/or shading topographic maps also show roads and other prominent features Thematic Map a tool to communicate geographical concepts such as the distribution of population densities, climate, movement of goods, landuse, etc.

4 Map as Model: The Abstraction of Reality Models are simplifications - not miniature versions of the reality Maps are a type of geographic model Maps must be abstraction from reality Purpose of Cartography Cartography is the art and science of mapmaking Communication is the traditional objective Analysis has become an important objective with the development of CAM and GIS

5 Map Scale Map scale defines the amount of reduction of reality It is the ratio between distances on map and corresponding distances in the real world Scale is expressed in three primary ways 1.Verbal Scale 2.Representative fraction (RF) 3.Graphic scale (bar)

6 Verbal Scale one inch equal (represents) to 63,360 inches eight inches equal 1 mile These are not the same as representative fraction

7 Representative Fraction Expressed as a ratio in the same units 1:2,000 means that one inch (or one meter) on the map represents 2,000 inches (or meters) on the ground You can develop verbal expression that may be useful to novice map users

8 Graphic Bar The graphic bar places visual measure of ground distances on the map Most software can automatically generate a graphic scale Used on printed maps (output of GIS) to aid in communicating the scale Remains accurate after mechanical enlargement of map, printed ratio or printed scale will be wrong after zooming the page on the copy machine

9 Scale Computations Scale Computation: Verbal to RF First, convert units to common units - inches - by multiplying 66 feet by 12 inches = 792 Since the units are common the RF is simply the ratio 1:792

10 Scale Computations Scale Computation: RF to Verbal First, compute the number of common units (inches) there are in the desired units (miles) by multiplying 5,280 feet by 12 inches = 63,360 Now divide 250,000 inches into miles You get miles Thus the Verbal scale is 1 Inch = miles

11 Scale Computations

12 Scale Computation: Graphic Scale Assume an RF of 1:20,000 How long (inches) would a graphic scale showing 1 miles be? First, compute the number of inches in a mile (12 * 5,280 = 63,360) Divide 63,360 by 20,000 = 2.6 inches The scale bar would be 2.6 inches long depicting 1 mile

13 MAP SCALE & PRECISION Scale defines the precision of the location and the level of detail Be careful when using small scale maps as input and then enlarging Enlargement does not mean more information Scale controls not only how features are shown, but what features are shown

14 SMALL vs LARGE SCALE SMALL SCALE MAP Small scale maps show large features On the RF scale of 1:250,000 representative feature is small i.e.,1/250,000 The ratio is small and the amount of reduction is large, producing a map of a large area LARGE SCALE MAP Large scale maps show great details On the RF scale of 1:10,000 representative fraction is large i.e.,1/10,000 Large scale means less reduction and a map covering a small area.

15 Scales of Measurement It is important to recognize the scales of measurement used in GIS data as this determines the kind of mathematical operations that can be performed on the data Following scales may define the Numerical values nominal ordinal interval ratio

16 Nominal Scale Numbers merely establish identity e.g. in the race, numbers issued to racers are used to identify individuals these numbers do not indicate any order or relative values in terms of race outcome common operations are frequency and aggregate totals

17 Ordinal Scale On ordinal scale numbers establish order only e.g. in the race, the finishing places of each racer are measured on ordinal scale however we do not know how much time difference there is between each racer common operations are median and percentile

18 Interval Scale On interval scale, the difference (interval) between numbers is meaningful, but the numbering scale does not start at zero subtraction makes sense but division does not e.g. in the race, if the racers finished at 9:10, 9:20 and 9:25 IST respectively, then racer 1 finished 10 minutes before racer 2 and the difference between racer 1 & 2 is twice that of the difference between racer 2 & 3 however, the racer finished at 9:10 did not finish twice as fast as the racer finishing at 18:20 common operations are arithmetic mean, standard deviation and correlation and regression analysis

19 Ratio Scale On a ratio scale, the measurement has an absolute zero and the difference between numbers is significant division makes sense e.g. in the race, if the 1st place finisher finished in a time 2 hrs 30 min, the 2nd in 2 hrs 40 min and 150th place finisher took 5 hrs; the 150th place finisher took twice as long as the 1st place finisher (5/2.5) all mathematical operations with real numbers

DATA VISUALIZATION GABRIEL PARODI STUDY MATERIAL: PRINCIPLES OF GEOGRAPHIC INFORMATION SYSTEMS AN INTRODUCTORY TEXTBOOK CHAPTER 7

DATA VISUALIZATION GABRIEL PARODI STUDY MATERIAL: PRINCIPLES OF GEOGRAPHIC INFORMATION SYSTEMS AN INTRODUCTORY TEXTBOOK CHAPTER 7 Contents GIS and maps The visualization process Visualization and strategies