1 Practical Vector GIS Globe to map

2 The where is it… How do we locate Syracuse in space on the earth’s surface? On a FLAT surface?

3 The where is it… How do we locate Syracuse in space on the earth’s surface? On a FLAT surface?

4 Syracuse W 43.07N  How do we locate Syracuse on earth? degrees west of meridian through Greenwich, England degrees N of the equator

degrees west of meridian through Greenwich, England degrees N of the equator

6 Earth’s Coordinate System X is Longitude and is measured E and W from Greenwich, England. West is negative, East is positive Y is latitude and is measured N and S from the equator. North is positive and S is negative. These are called Geographic Coordinates North Pole South Pole Lat = 0º Lat = -30º   Long = -6 0º  Lat = 30º

7 Coordinates on Earth North Pole South Pole Latitude Longitude Equator Meridians Parallels

8 X, Y = Longitude, Latitude Lines of constant Longitude Lines of constant Latitude Equator Stretch the top Stretch the bottom

9 X, Y = Longitude, Latitude Lines of constant Longitude Lines of constant Latitude Equator 90E, 30N 90W, 30S +90, W76.15° N43.04°-76.12° 43.08°

10 The world in Geographic Coordinates Is Antarctica Really that big?

11 The where is it… How do we locate Syracuse in space on the earth’s surface? On a FLAT surface?

12 The where is it… How do we locate Syracuse in space on the earth’s surface? On a FLAT surface? What we just did, plot Long, Lat coordinates, put the globe on a flat surface but  DISTORTION Why distorted?

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17 Maps are Flat The globe is an ideal model of the earth (almost) But you can’t put a useful one in your pocket  usless So the problem is to put data from a sphere (almost) onto a flat surface Xerox can’t do it

18 Overview 1.There are a number of problems that apply when converting to flat maps Geographic coordinate systems #1 problem – Datums #2 problem – Projection #3 problem – Scale #4 problem – Generalization 2.Here they are, 1 by 1

19 Problem #1, Datums Earth is NOT a sphere! It is more pear shaped To accommodate this geographers and surveyors have created models of the earth’s surface These are called Datums And this is booby trap #1 because… Different shapes  different coordinates

20 Trees Don’t Move Much But their coordinates can change The Long/Lat of this tree will be different depending on which datum is being used! Could be up to ~50m different in the US There are lots of different datums to contend with!

21 Problem #2

22 Mercator

23 The Projection Problem There are many mathematical ways of projecting the spherical surface onto a flat surface. For the earth these have names like Mercator Albers Polyconic Lambert equal area Azimuthal Peters Albers equal area

24 Other Projections Wrong Question – they are all right, just different. And they all have different properties

25 Problem #3 Geographic SCALE

26 Definition Scale = distance on map(distance unit) distance on ground (distance unit) A Scale of 1/24,000 means 1 inch (or foot, or furlong) on the map = 24,000 inches (or feet or furlongs) on the ground.

27 2,600 Mi 3.5” Numeric or Ratio scale =1/47,067,429

28 Living Room Kitchen Dining Room 2.6” / 25’ Scale = 1/115

29 Living Room Kitchen Dinning Rm. Scale 1/47,000,000 1/46,000 1/115 Is a smaller number than = = = Small Scale data Large area/sheet Least accurate Large Scale data Small area / sheet Most accurate

30 Living Room Kitchen Dinning Rm. Scale 1/47,000,000 1/46,000 1/115 Is a smaller number than = = = Small Scale data Large area/sheet Least accurate Large Scale data Small area / sheet Most accurate 8888

31 Problem #4 -Accuracy & Generalization When a paper map is made at a very small scale the cartographer is limited by the pen being used Can’t draw anything finer than the width of the pen line. At a scale of 1/1,000,000 a line 0.05 cm wide = 0.05x1,000,000 cm or 50,000 cm or 500 meters or 19,850” or 1,640’ wide! What road is 1,640’ wide!!! So on the map the road is much, much too wide

32 Accuracy & Generalization Take the case of a winding stream Shrink it to a Smaller scale (large area, small paper Now it is hard to see what is there So the cartographer simplifies the stream

33 Accuracy & Generalization The generalized stream is not as accurate a representation of the stream as the original And if you try to mix data of different scale common lines are NOT going to match Original Generalized

34 Booby Trap Summary Using a GIS is more than just combining various data layers – just knowing what buttons to push is NOT sufficient!!! You have to be careful that the basic 4 booby traps outlined above do not cause problems And 4 possible sources of error give Murphy a field day since problems encountered go up as n 2

35 Booby Trap Summary Using a GIS is more than just combining various data layers You have to be careful that the basic three booby traps outlined here do not cause problems And 3 possible sources of error give Murphy a field day since problems encountered go up as n 2 Datum Projection Scale Generalization

36 iceberg And that was just this! This topic will be a major part of the course!