GEO381/550 Lecture Notes of September 2, 2004

The map defined

Map = graphic representation of the milieu

 

Map is a model of reality

Source: USGS National Atlas Shaded Relief Map

 

  1. Reduce areas on the earth fitted to small sheet of map à map scale
  2. Flatten curved earth to the flat map à map projections
  3. Abstract geographical reality through selection, and symbolization à map symbols (cartographic abstraction and generalization)
    1. select relevant features: does not show everything only shows relevant features

a.      generalize features: simplify unnecessary details

b.      emphasize/eliminate a specific feature depending on map purpose

c.      symbolize features (e.g. markers used for indicating school or church)

 

Three elements of maps: (1) scale (2) map projections (3) map symbolization

They are interdependent: e.g. scale determines the degree of generalization, scale is decided upon map projections. 

 

*Requirements of good map

  1. choose scale such that general trends/pattern can be pronounced while relevant specifics are not masked, not to mention it has to show the complete (geographic) area of interest
  2. choose map projections such that the distortion can be minimized and serves the purpose of map (e.g. navigation, thematic mapping)
  3. Choose map symbols such that the gap is reduced between what map authors are intending to convey and what map viewers can interpret by associating meanings with the symbol. Often the association relies on (1) convention (e.g. interstate shield; look the map below) (2) intuition (e.g. dark shade linked to high magnitude of value) or (3) legends

 

 

Source: USGS National Atlas  

Power of map

Maps are worth a thousand words

 

Unlike other modes of communication (e.g. word, number), maps are visual. It is an “effective” way of conveying message – fast, and retentive

Census 2000 Gateway: http://www.census.gov/main/www/cen2000.html

e.g. compare the table showing median age by state to choropleth map

When to use maps versus when not to use maps?

  1. want to know “overall” spatial distribution of variables of interest à go to map
  2. want to know exact value of each enumeration unit à refer to the list of values

For example, classification of value masks details

You may need to provide the relevant numeric values if necessary along with map.

Kinds of map (D1:6-10)

 

Compare Figure 1.4 to Figure 1.5

1. Reference map (general-purpose map): show location of multiple features

2. Thematic map (single-purpose map): show topical subject

a. qualitative: nominal data (e.g. administrative boundary, agricultural region) Fig 1.6

b. quantitative: numerical data (e.g. population density, income) Fig 1.7

The boundary between reference map and thematic map is not necessarily crisp. Often the division relies on convention.

 

Classification of kinds of maps (Figure 1.3)

 

Quiz: what is reference map, and what is not reference map? If it is thematic map, is it qualitative or quantitative?

 

Distribution of urban and rural population

Source: USGS National Atlas

 

Topographic map

Source: USGS National Atlas

 

Distribution of Indian tribes & language

Source: USGS National Atlas

 

Island of Kauai

Source: Old map gallery

 

Scanned Image of 24K topographic map (a.k.a USGS DRGs (Digital Raster Graphics))

Source: USGS

 

Southern California Map

Source: Unique Media Inc.

 

New York Downtown’s contribution in fund race 2004

 

Source: Fund Race 2004

 

URL

U.S. National Atlas: http://nationalatlas.gov/atlasvue.html

Odden’s Bookmarks: http://oddens.geog.uu.nl/index.php

Old Map Gallery: http://www.oldmapgallery.com/catalog/

USGS DLG http://topomaps.usgs.gov/drg/

Fund Race 2004: http://www.fundrace.org/citymap.php

 

Does it portray a consistent topic?, does it show spatial distribution of a particular phenomenon or just show the location of multiple features?, the amount of qualitative information presented

Component of thematic map (D1:10-11)

 

Figure 1.10

(1)   base map: can be extracted from reference map

(2)   thematic overlay: can be collected from survey, existing database, and so on

Relevant base map should be chosen.

Map scale (D1:11-12, D2:36-37, M2:5-8)

 

0. how to read scale ratio?

See Plate 14 – look at scale ratio

Where left-hand side represents dimension on the map and right-hand side represents dimension on the earth; both are in the same unit

(d) 1:1,000,000 means 1 dimension on the map represents a 1,000,000 dimension on the mapped earth; 1-inch line on the map represents 1,000,000-inch (equivalent to 15.78 miles) stretch on the earth

Do the same for (c)

 

1. what is scale?

= map distance / earth distance

Tells us how much is reduced when transforming the earth area into the map plane

 

2. three customary ways of expressing scale on a map

 

(1) representative fraction (RF)  1:25,000

(2) graphic bar scale                   

(3) verbal scale                                         “one centimeter represents 10 kilometers”

 

Q. which methods do you think will be robust particularly when you have image files that will be zoomed in and out?

 

3. convert RF to verbal scale, and vice versa

 

Eg1) 1:250,000

Plate 14 (c)

1 centimeter on the map represents ? kilometers on the earth

1 inch on the map represents ? miles on the earth

Where 1 mile equals 63360 inches

 

Eg2) 1 inch to 4 miles (where 1 mile equals 63,360 inches)

4 miles = ? inches

Therefore, scale ratio will become 1:253,440

 

Eg3) 4 inches to the mile

4: 63,360 = 1: x

x = 15,840

Therefore, scale ratio will become 1:15,840

 

Eg4) The distance between two known points on a map is 5 miles. What is the scale of a map on which the points are 3.168 inches apart? (text p.37)

 

3.168 inches to 5 miles

5 miles equals 316,800 (=5*63,360)

3.168:316,800 = 1:x

x = 100,000

Therefore, scale ratio will become 1:100,000

 

Eg5) Look at Plate 14 (b). You know the scale is 1:100,000 from the bottom. Somebody asks you “what will be the real distance (in miles) between city hall and ③ when you drive along Main St?” Answer him or her.

 

4. Small scale map versus large scale map

 

USGS DLG Product information: http://edc.usgs.gov/products/map/dlg.html#description

 

Small-scale map: < 1:30,000,000 show large earth area

Large-scale map: > 1:24,000 show small earth area in detail

Intermediate-scale map: in-between

But no rigorous numerical boundaries; rather they are relative

Confusing? But remember 1/30,000,000 < 1/24,000

 

Loot at Plate 14: which is mapped in a smallest scale? (a) or (d)?

 

Map scale and its effect on mapped earth area, map information, and symbolization (Figure 1.11)

 

Map scales in paper map versus GIS: zoom in & zoom out

 

Scale is fixed in paper map, but scale varies in GIS as you zoom in/out

Note the spatial data itself are derived from paper map or other sources

In the metadata, read data preprocessing part (mostly in identification part).

 

Example of spatial data derived from small-scale map (e.g. National hydrological unit):

http://nationalatlas.gov/hucsm.html#1

Example of spatial data derived from large-scale map (e.g. school district):

http://www.nysgis.state.ny.us/gis3/data/orps.schooldist00.html#Identification_Information

Map projections

 

Peel and flatten the earth like orange peel

 

Visualize projecting a light through the Earth onto a surface (called the projection surface)

 

1. Can be classified by

 

(1) Which shape of the projection surface is used: cone, cylinder, plane

 

Conic projection

 

Cylindrical projection

 

Planar

 

(2) Where to contact: point of contact (tangent), line of contact (secant)

 

Tangent

 

Secant

 

(3) From where the light source is emanating: center, surface, infinite point (only applied to planar projections)

 

 

2. Projection methods determine the geometric properties of resulting maps

(1) conformal

(2) equal-area

(3) equidistance

(4) true direction

 

3. Map projections will be chosen depending on

(1) map purpose: thematic mapping, navigational maps, and so on

(2) the extent of the area to be mapped: world, hemisphere, continent or small-region

(3) location of the earth to be mapped: polar, mid-latitude, or equatorial region

 

4. Coordinate system

Defined by datum, map projection, and so on

Can be divided into (1) universal CS (e.g. UTM) (2) local CS (e.g. SPC)

You can define your own coordinate system too.

 

Real-world problem, suppose you are working as a GIS analyst, and you obtain the data set. How do you know in which coordinate system the data is stored?

Check metadata – spatial_reference_information.

http://www.nysgis.state.ny.us/gis3/data/orps.schooldist00.html#Spatial_Reference_Information

 

What about working on dataset from different coordinate systems?

e.g. school district boundary in UTM and state boundary in geographic coordinate system.

Cartographic abstraction and generalization (D1:15-17)

 

How you make the balance between specifics and general; suppress unimportant information and make important information pronounced; choose a symbolic code based on an understanding of graphic logic and the limitations of visual perception

 

Consists of the following four parts

  1. Selection

Choose where to map, how much to be reduced (scale), map projections, data variables, and so on

  1. Classification

Reduces the complexity of the map image, thus enhances communication

a. Qualitative (e.g. agricultural region)

b. Quantitative (e.g. housing price)

  1. Simplification (Figure 1.14)

Compare large-scale cartographic boundary file to small-scale cartographic         boundary file

  1. Symbolization

Since it is not possible to create a reduced image of the real world, cartographers devise a set of marks (symbol) that stand for real-world things.

    1. Replicative: looks like real-world features (e.g. lake, road)
    2. Abstract: does not look like real-world features (e.g. circles, triangles)

Good match between data (e.g. point, line, area) and symbol

Good match between measurement (e.g. quantitative, qualitative) and symbol

The role of map (D1:12-15)

 

1. Help map readers identify unknown pattern (e.g. correlation between racial diversity and housing price: good research tool); let map readers gain a new spatial knowledge; high degree of human-map interaction; cognitive interaction; more leeway for map readers

2. Convey message; present knowns; low degree of human-map interaction 

 

Illustration for 1

              Correlation between racial diversity and housing price?

 

Illustration for 2

              M7:95 Figure.7.6

 

A model of map visualization and communication (figure 1.12)

  1. Map visualization: private realm, visual thinking, unstructured symbol, map users
  2. Map communication: public realm, visual communication, structured symbol, map authors

 

Where should a map design be situated?

What is map design? (D1:17-18)

 

A model of map communication (figure 1.13)

 

Should be aimed at reducing the discrepancy between what map authors intend to convey and what map readers interpret

Should be able to take into account what users want, and what users already have known

  1. involves specification of the selection of scale, map projections, symbols, and so on
  2. is intellectual and visual
    1. intellectual in that it relies on the foundations of sciences (communication, geography, psychology)
    2. visual in that it communicates through a visual medium

Aim is to achieve the efficient and accurate transfer of knowledge between map author and map user