GIST 3300 / 5300
Geographic Information Systems
Geographic Coordinate System or Grid
Geographic Information Systems
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GIST 3300 / 5300
Geographic Information Systems
Last Time
Data Classification Methods
Take home point: a map is a communication medium and
you as the cartographer or GIS analyst can manipulate
the appearance of a map to reinforce a point of view
using different classification methods
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Geographic Coordinate System or Grid
- Geographic Coordinate System
- Cartesian vs spherical coordinate system
- properties of latitude (parallels)
- properties of longitude (meridians)
- longitude and time
- describing coordinates
- latitude and longitude in a GIS
- distinguishing hemispheres in a GIS
Geographic Information Systems
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Geographic Coordinate System or Grid
GIS represents reality, but it is not reality
- To be useful GIS maps need to accurately represent locations of things or
features in the real world on a map
- To do this we need a standard framework to help us define locations
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Geographic Coordinate System or Grid
- We call these frameworks reference systems
- Reference systems are a set of lines of known location that can be used
to determine the locations of features
- In a GIS we call these reference systems:
coordinate systems
By understanding coordinate systems,
you will be able to manage your data in a
way that increases the accuracy of your
GIS maps and all the work you do with
them.
Geographic Information Systems
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Geographic Coordinate System or Grid
- if the earth was flat …
- we could use a Cartesian coordinate system
Cartesian coordinates describe locations on a flat or planar surface
(0,0)
Rene Descartes
Cartesian coordinate system
- specifies each point uniquely in a
plane by a pair of numerical
coordinates
- which are the signed distances to the
point from two fixed perpendicular
directed lines
- measured in the same unit of length
(0,0)
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Geographic Coordinate System or Grid
- But the earth is not flat – so Cartesian coordinates do not work
- So we need a different type of reference system to locate
places on the spherical earth
- The network of intersecting lines (longitude and latitude lines)
form the geographic coordinate system or geographic grid
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Geographic Coordinate System or Grid
Geographic Coordinate System = synonymous with Geographic Grid
Geographic Grid – a reference system or location framework to locate
features on earth
A term for the set of intersecting
latitude and longitude lines is a …
graticule
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Geographic Coordinate System or Grid
-
based upon angular measurements (not planar)
measurements expressed in degrees, minutes and seconds
A degree is 1/360 of a circle
each degree can be divided into 60 minutes
each minute can be divided into 60 seconds
To describe the location
of Peoria, Illinois we
would say it is at 40
degrees N. Latitude and
90 degrees W.
Longitude
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Geographic Grid
Spherical coordinate system
- these degrees are measured as angles from the center of the earth to a
point on the surface. That is why we call latitude/longitude angular coordinates
So from the Center of Earth at the
equator, if we go up 30 degrees
north we would be at New Orleans.
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Geographic Grid
Latitude (parallels)
Definition: east-west lines used to measure the arc distance north and
south of the Equator
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Geographic Grid
Latitude (also called parallels)
- properties
- are parallel to one another
- encircle the globe
- can range from 0 to 90 degrees N & S
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Geographic Grid
Important Lines of Latitude
North Pole 90o
Arctic Circle 66.5o N
Tropic of Cancer 23.5o N
Equator 0o
Tropic of Capricorn 23.5o S
Antarctic Circle 66.5o S
South Pole 90o
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Geographic Grid
Longitude (also called meridians)
Definition: north-south lines used to measure the arc distance east
and west of the Prime Meridian
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Geographic Grid
Longitude (meridians)
- properties
- converge at the poles
- do not encircle the globe
- range from 0 to 180 degrees E & W
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Geographic Grid
Two important lines of longitude
- the Prime Meridian
- 0o of longitude
- runs through the Royal Observatory in Greenwich, England
- International Dateline
- 180o of longitude
- runs through the Pacific Ocean opposite Greenwich
Western
Hemisphere
Geographic Information Systems
Eastern
Hemisphere
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Geographic Grid
The Prime Meridian (today) – is only one of many historic PMs
Washington, D.C. (77° 3′ 2.3″ W; Washington Meridian)
Philadelphia (75° 10′ 12″ W)
Canary Islands (18° 03' W)
Lisbon, Portugal (9° 07′ 54.862″ W)
Madrid, Spain (3° 41′ 16.58″ W)
Prime Meridian (0o) Greenwich, England
Paris, France (2° 20′ 14.025″ E; Paris Meridian)
Brussels, Belgium (4° 22′ 4.71″ E)
Berne, Switzerland (7° 26′ 22.5″ E)
Pisa, Italy (10°24′ E)
Oslo (Kristiania), Norway (10° 43′ 22.5″ E)
Rome, Italy Meridian of Mount Mario (12° 27′ 08.4″ E;)
Copenhagen, Denmark (12° 34′ 32.25″ E)
Stockholm (at the observatory), Sweden (18° 3′ 29.8″ E)
Warsaw, Poland (21° 00’ 42” E; Warsaw Meridian)
Oradea, Romania (21° 55′ 16″ E)
Alexandria, Egypt (29° 53' E)
Saint Petersburg, Russia Pulkovo Meridian (30° 19′ 42.09″ E)
Great Pyramid of Giza (31° 8′ 3.69″ E, 1884)
Jerusalem (35° 13′ 46″ E; the large dome of the Church of the Holy Sepulchre)
Mecca (39° 49′ E)
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Map of Texas
1816
Longitude West from Washington
24 Longitude 23 West
22
from
21 Washington 20
19
“Longitude West from Washington”
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260
270
280
290
300 …. degrees
This map starts at some 0 degree
meridian and keep going to
360 degrees around the world
The point is the notion of a PM location is arbitrary – so are hemispheres
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Geographic Grid
The Prime Meridian
- what we use today is an arbitrary meridian
- runs through the Royal Observatory in Greenwich, England
- first established by Sir George Airy in 1851
- by 1884 two-thirds of all ships used it as the reference on their maps
- international conference held in Washington, D.C. in 1884
- delegates represented 25 nations – prominent sea-faring nations
- delegates selected the Greenwich Meridian owing to its wide use
- only France abstained
- used the Paris Meridian for several more decades
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Geographic Grid
The Prime Meridian
- located outside of London
Royal Observatory, Greenwich, England
Prime Meridian
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Geographic Grid
The Prime Meridian
- located outside of London
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Geographic Grid
Longitude and Time
- the Earth rotates on its polar axis once each day
o
- the rotation covers 360 in 24 hours
o
- if we divide 360 / 24 hours
- the rotation covers 15o / hour
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Geographic Grid
- this is the basis for the 24 world time zones
- each time zone covers 15o of longitude (360 / 24 )
International Dateline
180o of longitude
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Greenwich Mean Time
0o of longitude
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Geographic Grid
Describing Coordinates
When describing coordinates, by convention, we usually give the
latitude (N or S) first, followed by the longitude (E or W).
For greater precision, each degree can be subdivided into 60
minutes and each minute can be subdivided into 60 seconds.
A typical geographic coordinate might then be expressed as:
24 15' 30" N, 110 38' 12' W.
(note: if convert geographic coordinates to Cartesian coordinates
latitude represents y-axis and longitude represent x-axis)
Remember - Geographic coordinates describe angular distances
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Geographic Grid
Latitude and Longitude in a GIS
In a GIS, degrees of latitude and longitude are expressed as
decimal degrees. A computer cannot work with data entered in
degrees, minutes and seconds.
To convert degree/minute/second coordinates to decimal degrees:
1) divide the seconds by 60 (decimal minutes),
2) add this fraction to the minutes,
3) and then divide this sum by 60.
24 15' 30"
24 15' (30"/ 60”) = 24 15.5‘ (decimal minutes)
24 (15.5'/ 60’)
Geographic Information Systems
= 24.2583 (decimal degrees)
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Geographic Grid
Distinguishing Hemispheres in a GIS Environment
It is also important to note that positive and negative values are assigned
to coordinates in a GIS to distinguish between the hemispheres.
With respect to LATITUDE,
- northern hemisphere coordinates are positive (+)
- and southern hemisphere coordinates are negative (-).
With respect to LONGITUDE,
- eastern hemisphere coordinates are positive (+)
- and western hemisphere coordinates are negative (-).
0o
W
N
24.2583, -110.6367
lat itude
Y
longitude
X
Geographic Information Systems
0o
S
E
+−
++
−−
−+
lat, lon
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Housekeeping
Fun with the Esri e-Learning site
If you are having difficulty accessing the homework assignments:
- Log out of the website
- Completely close your browser
- Open a new browser
- Log in again to refresh your session
You should be able to access the e-Learning assignments
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Housekeeping
Uploading Lab Assignments to your Student Folder
Make sure you upload a
PDF or JPG as assigned
If you upload a .MXD alone that won’t work!
The data are not stored in the MXD.
The data are stored separately on disk!
TAs can open the MXD, but they won’t see any of your layers since
their path will be different than yours.
Remember an .MXD is a document that only references the data on
disk (though it’s pathname to the data).
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GEOGRAPHIC INFORMATION SYSTEMS
Lecture 06: Geographic Coordinate System or Grid
The Geographic Coordinate System or Grid
- if the Earth was flat we could use a Cartesian (x,y) coordinate system to locate places on the surface
- because the Earth is a spheroid, this approach doesn’t work – so we must use a spherical coordinate system
- the geographic grid is a spherical coordinate system based on latitude and longitude
- lines of latitude and longitude are measured using degrees, minutes and seconds (angular measurement)
- these angular measurements for latitude and longitude are referenced from the center of the Earth
Latitude (also called parallels)
- east west line used to measure the arc (angular) distance north and south of the Equator
- range from 0° to 90° north and south of the Equator
- encircle the globe (except for north and south poles)
- lines of latitude are parallel to one another
- important lines of latitude include:
- North Pole (90°N) and South Pole (90°S)
- defined by the Earth’s polar axis of rotation
- Equator (0°)
- the latitude that bisects the Earth’s polar axis of rotation
- Tropic of Cancer (23.5°N) and Tropic of Capricorn (23.5°S)
- the most northerly and southerly latitude where the sun is directly overhead (90o)
- on an annual basis, the declination of the sun varies between the two tropics
- Arctic Circle (66.5°N) and Antarctic Circle (66.5°S)
- the most northerly and southerly latitude where the daylength can be 0 or 24 hours
- defined by the Circle of Illumination
Longitude (also called meridians)
- north-south lines used to measure the arc (angular) distance east and west of the Prime Meridian
- range from 0° to 180° east and west of the Prime Meridian
- lines of longitude do not encircle the globe (they extend from pole to pole)
- meridians converge at the north and south poles
- important lines of longitude include:
- Prime meridian in Greenwich, England (0°)
- International Dateline - mid Pacific (180°)
- Prime Meridian – only of one of many that have been used over the years
- runs through the Royal Observatory in Greenwich, England
- the Prime Meridian was established by an international convention in 1884
Longitude and Time
- the earth rotates 360° in 24 hours or 15° per hour
- this is the basis for the world’s 24 time zones where (ideally) each time zone covers 15° of longitude
Describing Coordinates
- by convention, we usually give latitude first, followed by longitude
- for greater precision, each degree of latitude or longitude can be subdivided into 60 minutes
- for still greater precision, each minute can be divided into 60 seconds
example: 24° 15' 30" N, 110° 38' 12' W.
Latitude and Longitude in a GIS
- in a GIS, latitude and longitude are expressed as decimal degrees
- know how to convert degrees, minutes and seconds to decimal degrees
- know how to differentiate the hemispheres as positive or negative (+, -) decimal degrees
- e.g. western and southern hemispheres are expressed as negative values
same example expressed in decimal degrees: 24.2583, -110.6367
Demonstration 1: Showed a map document of the world with the North Pole in the center.
Copyright© 2016, Kevin Mulligan, Lucia Barbato, Texas Tech University
The Prime Meridian was shown running through Greenwich England so the 0° meridian runs from the center of
the map (North Pole) to the bottom of the map (South Pole). On the top of the map is a line running from the
North Pole to the top of the map labeled 180°. This meridian is the International Date Line. The IDL is an arbitrary
or imaginary line on the surface of the Earth that runs from the North Pole to the South Pole and demarcates the
change of one calendar day to the next. For the most part the IDL runs through the middle of the Pacific Ocean
roughly following the 180 degree line of longitude but deviates to pass around some territories and island groups.
If you cross the IDL westbound you add 24 hours to your clock advancing the calendar date by one date. If you
are traveling eastbound and cross the IDL you must subtract 24 hours and move the date back one.
Notice: with the PM at the bottom of the screen the eastern hemisphere is on right and the western hemisphere is
on the left – as you would customarily see on most maps. (E to W)
Demo: Customize > Toolbars > Data Frame Tools >. Rotate the Earth 180 degrees so that the PM is at the top.
When the PM at the top of the map notice that the hemispheres are switched, so that the eastern hemisphere is
on the left. Kind of cool!
If time:
Demonstration 2:
MapScale map document to show how scale works on a world map.
Representative Fraction or Absolute Scale
• The map was set up on a 40” x 30” piece of paper in the Layout View where we can add map
elements to create our map. In Lecture 3 we talked about bar scales, verbal scales and
representative fraction. In lab you might have already seen a bar scale.
• In ArcMap we can insert different types of scales. Insert > Scale Text > Absolute Scale > OK
(means the same as a representative fraction or RF).
• The absolute scale should display at the same as shown the scale display area in ArcMap.
• If the scale display is changed to 1:100M. The RF changes as well.
The problem is what if you want to measure a distance in Kilometers? Not so easy. Have to do a
little math to convert say inches to km.
Scale Bars
Copyright© 2016, Kevin Mulligan, Lucia Barbato, Texas Tech University
•
•
•
There are many different scale bar formats available: Insert menu> Scale Bar > Scroll through the
options.
Add another scale bar: Alternating Scale Bar 2 > Properties > Adjust width > division value 5000 >
number of divisions 4 > Subdivisions 0> Units KM. Numbers and Marks > frequency > divisions.
Format tab > Change font to Arial 48 and size to 18 point.
Demo 2
Data has a scale associated with it
Displayed another map document with Texas counties. The first map has a scale of I: ~9M. It is also
important to consider that data we see are also created at a specific scale. So keep in mind that the
map has a scale and also the data has a scale associated with it.
• The TXCnty_Esri data are very generalized. Zoomed to Galveston Coast. Coast lines appears
straight and jagged – not very detailed. These data were created at a scale of 1:1M and not
intended for use at scales much larger than that.
• TxCounties_USGS layer is slightly more detailed at the same map scale. The data were digitized at
a scale of about 1:50,000.
• StratMap layer. StratMap or Strategic Map of Texas data are extremely detailed data at the same
map scale. The data were digitized at a scale of 1:24,000.
• So when you are searching for data it is important to work with data that was developed at a scale
that is appropriate to the type of mapping work you are doing.
Copyright© 2016, Kevin Mulligan, Lucia Barbato, Texas Tech University