Practical Vector GIS
Globe to map
1
The where is it…
• How do we locate Syracuse in space
on the earth’s surface?
• On a FLAT surface?
2
The where is it…
• How do we locate Syracuse in space
on the earth’s surface?
• On a FLAT surface?
3
-76.19 degrees west
of meridian through
Greenwich, England
43.07 degrees N of
the equator
How do we locate Syracuse on earth?
4
43.07 degrees N of the equator
-76.19 degrees west of meridian through
Greenwich, England
5
Earth’s Coordinate System
North Pole
Lat = 30º
Long = -6 0º
Lat = 0º
Lat = -30º 
South Pole
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
6
Coordinates on Earth
North Pole
Meridians
Longitude
Equator
Parallels
Latitude
South Pole
7
X, Y = Longitude, Latitude
90
60
30
0
-30
-60
-90
-90Stretch the
0 bottom
+90
+180
Lines of constant Longitude
Lines of constant Latitude
Stretch the top
Equator
-180
8
X, Y = Longitude, Latitude
90E, 30N
Equator
90W, 30S
-180
+90, +3090
60
30
0
-30
-60
-90 -30
-90
-90
0
+90
+180
Lines of constant Longitude
Lines of constant Latitude
9
The world in Geographic
Coordinates
Is
Antarctica
Really that
big?
10
The where is it…
• How do we locate Syracuse in space
on the earth’s surface?
• On a FLAT surface?
11
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?
12
13
14
15
16
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
17
Overview
1. There are a number of problems
that apply when converting to flat
maps
•
Geographic coordinate systems
•
•
•
•
#1
#2
#3
#4
problem
problem
problem
problem
–
–
–
–
Datums
Projection
Scale
Generalization
2. Here they are, 1 by 1
18
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
19
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!
20
Problem #2
21
Mercator
22
The Projection Problem
• There are many mathematical ways of
projecting the spherical surface onto
a flat surface.
• For the earth these have names like
Albers equal area
Mercator
Albers
Peters
Polyconic
Lambert equal area Azimuthal
23
Other Projections
Wrong Question – they
are all right, just
different.
And they all have
different properties
24
Problem #3
Geographic
SCALE
25
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.
26
3.5”
2,600 Mi
Numeric or Ratio scale
=1/47,067,429
27
2.6” / 25’
Scale = 1/115
Living Room
Dining Room
Kitchen
28
Scale
1/47,000,000
=0.000000021
Is a smaller
number than
Small Scale
data
Large
area/sheet
Least accurate
1/46,000
=0.000022
Is a smaller
number than
Living Room
Dinning Rm.
Kitchen
1/115
=0.008696
Large Scale
data
Small area /
sheet
Most accurate
29
Scale
1/47,000,000
=0.000000021
Is a smaller
number than
Small Scale
data
Large
area/sheet
Least accurate
1/46,000
=0.000022
Is a smaller
number than
Living Room
Dinning Rm.
Kitchen
1/115
8888
=0.008696
Large Scale
data
Small area /
sheet
Most accurate
30
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
31
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
32
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
33
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 n2
34
Booby Trap Summary
• Using a GIS is more than just
combining
various data layers
•Datum
• You have to be careful that the basic
three booby traps outlined here do
•Projection
not cause problems
• And 3 possible sources of error give
•Scale
Murphy a field day since problems
encountered go up as n2
•Generalization
35
And that was
just this!
iceberg
This topic will be a major part of the course!
36