Lecture Outline. Geography 12: Maps and Mapping. Review questions. Review questions. Large and Small Scale Maps. Large and Small Scale Maps

Transcription

1 Lecture Outline Geography 12: Maps and Mapping Lecture 3: Scale and Cognitive Maps Today s fun map: Review of Geodesy, Latitude & Longitude, Datums, and Reference Grids Scale Maps in the Head Review questions What is geodesy? What are the three main models of the shape of the earth? How did Eratosthenes estimate the size of the earth? How have computers influenced how latitude and longitude are formatted? Review questions What is a datum? What am I supposed to do with all of those abbreviations and numbers? Why are there so many datums? (and shouldn t that really be data and not datums?) What are the similar features of the UTM and State Plane reference grid systems? Large and Small Scale Maps Large and Small Scale Maps Often a source of confusion Scale can mean relative size of something The house was build on a grand scale With maps, the larger the extent, the smaller the map scale (generally) So 1:10,000 is larger scale than 1:100,000 Large and Small Scale is relative Sometimes referred to as fine and coarse scales Large Scale Small Scale 1

2 Scale Scale Types Graphic Scales Word Statement One inch on the map is equivalent to two miles Or just 1 inch to 2 miles May be Metric, English, Nautical units Representative Fraction (RF) Expressed: 1:24,000 or 1/24,000 Unit Free or Both Units the Same Scale Scale Types Digital Maps and 3D Rendering make Eye Height a kind of scale Graphic Scale (or Bar Scale) If map is enlarged or reduced, scale is intact Fixed scale Very common for maps covering only a limited area Often an extension scale for minor units/shorter distances Variable scale Very common for maps covering a very large area Sometimes called Latitudinal Scales for that reason Common Scales Common Scales 1:24,000 used for USGS Topographic maps 1 inch = 2000 feet 1:63,360 used on many British maps 1 inch = 1 mile 1:25,000 1:50,000 1:100,000 1:1,000,000 Scale: 1:24,000 1:50,000 1:63,360 1:100,000 1:250,000 1:500,000 1 in on map represents: 2,000 feet 4,166 feet 1 mile 1.6 miles 4 miles 8 miles 1 cm on map represents: 240 meters 500 meters meters 1 kilometer 2.5 kilometers 5 kilometers Metric / International Maps 1:1,000, miles 10 kilometers 2

3 Areal Scales (note: not Aerial) Graphical Areal Scales Example: 1 square inch = 25 square miles Calculated from linear scale If 1:24,000, the areal scale is expressed 1:24,000 2 Scale Conversion You must be able to convert Representative Fractions (RFs) to/from Graphic Scales to/from Word Statements Review RF is an abbreviation for a very common kind of scale. What does RF stand for? What is one benefit of the RF scale? What is one drawback? What scale is the 7.5 Topo Quad? At this scale, one inch equals how many feet? Scale Generation Calculating scale from a map Some maps, and especially aerial photographs will not have a scale You can generate a scale in at least three ways Basic Concept Take a known distance Measure how long it is on the map Put measured distance / known distance Divide units until 1 is on top, and numerator and denominator are in the same unit 3

4 The formula RF = MD / GD Only issue: different units feet, yards, miles; mm, m, km Known distances, calculate scale Bridge to cemetery on map = 126 mm Bridge to cemetery on the ground = 3024m RF = GD / MD = 126 / RF = 1: Scale Generation: Using Known Distances Some features are always built to the same specifications baseball fields are an example. Let s calculate the RF on the screen for this aerial photo, knowing that it s 90 feet between bases Scale Generation: Use Lines of Latitude Lines of latitude are fairly evenly spaced Question: Why not totally even? Spacing averages km or about 69 miles Use this as a known distance, and follow method 1 to determine scale Lines of Longitude can be used, but widths vary with latitude Scale Generation: Use Another Map Use another map to measure a feature common to both maps in real world units Use this as your known distance and follow Method 1 4

5 Known scale, calculate distances The paper strip-to-graphic scale trick Map distance = 300mm Map scale = 1: RF = MD / GD so GD = MD / RF 1/RF is the denominator GD = 300 mm x = m Move the paper to the graphic scale Read off distance Trick works along roads Bigger range Vertical Scale In many cases vertical scale is exaggerated Showing heights would be less instructive about features Vertical scale 1 =500 Or Vertical exaggeration = 4 times 5

6 Vertical Exaggeration Vertical Exaggeration V.E. and Maps as Data Storage V.E. and Impressions of Slope Presentation in Google Earth What looks more real, no VE or VE? BTW, let s look at applications of UTM in Google Earth It isn t all about accuracy Slopes look steeper than they are 6