Chapter 3 - Geodesy, Datum, Map Projections & Coordinate Systems Flashcards
These define the location and shape of geographic entities
Spatial coordinates
What are two things that complicate the process of defining coordinates?
- Earth is best understood with flat maps and Cartesian coordinates; 2. Earth is not a perfect sphere shape
These are two ways to address the problem of defining coordinates
Geodesy and map projections
This is a discipline concerned with measuring and representing the Earth
Geodesy
What is geodesy needed to do accurately?
To locate points on a globe
What was geodesy originally based on?
Land surveys
What are land surveys combined with in modern geodesy?
Satellite telemetry
Geodesy creates these two shapes for reference used for spatial positioning
Ellipsoid and geoid
Humans have long speculated on this characteristic of the Earth
Shape
What are three of five shapes considered for the Earth over human history?
Disc, box, sphere, pear, flat
The Earth has often been seen as this shape
An infinite sea covered by a hemisphere of stars
Who are four of seven philosophers key to the history of earth measurement?
Pythagoras, Plato, Aristotle, Eratosthenes, Posidonius, Ptolemy, Newton
This philosopher postulated a spherical Earth, based purely on aesthetic reasoning
Pythagoras
This philosopher postulated a published and widely accepted idea of a spherical planet for aesthetic reasons
Plato
This philosopher posited the Earth was a sphere for geometric reasons and provided physical evidence
Aristotle
For how long was Aristotle’s theory of a spherical Earth generally accepted?
700 years
This philosopher used angles of shadows to measure the circumference of the Earth
Eratosthenes
By what percentage was Eratosthenes off in his measurement of the Earth’s circumference?
1%
This philosopher used star sightings to estimate the Earth’s size
Posidonius
By what percentage was Posidonius off in his estimation of the Earth’s circumference?
18%
This philosopher revised Posidonius’ estimation of the Earth’s size
Ptolemy
By what percentage was Ptolemy off in his estimation of the Earth’s size?
28%
Ptolemy’s estimation of the Earth’s size was used during this historical period
Renaissance
This scientist posited that the Earth’s rotation created an ellipsoid
Newton
When was Newton’s theory of the Earth’s shape proven?
1730
In what century did serious efforts to measure the Earth arise?
18th century
What are two reasons why efforts to measure the Earth rise in the 18th century?
Rising need for accurate maps and availability of better instruments
For how many years were Earth radii repeatedly measured starting in the 18th century?
Over 200 years
What two methods were combined to measure the Earth since the 18th century?
Posidonius method and land surveys
18th century Earth measurements used these to approximate the true shape of the Earth
Reference ellipsoids
An ellipsoid is described by these two axes
Semi-minor axis and semi-major axis
What three things are defined on a reference ellipsoid?
Radii, poles and equator
What are three reasons why so many reference ellipsoids of the Earth were created?
- Surveys needed points 30 miles apart; 2. Different methods were used between regions with different access to computers; 3. Earth is not a perfect ellipsoid, so an average of many is needed
These two types of modern technology have enabled precise measurement of the Earth
Satellites and lasers
What are the three current ellipsoids?
- World Geodetic System 1972; 2. Geodetic Reference System 1980; 3. World Geodetic System 1984
These two types of lines form a geographic coordinate grid lain over a reference ellipsoid
Latitude and longitude
These coordinate lines describe positions north-south of the equator
Latitude
These coordinate lines describe positions east-west of a reference line
Longitude
What are latitude lines called?
Parallels
What are longitude lines called?
Meridians
In what year was the prime meridian set?
1884
What is another name for the latitude-longitude grid?
Graticule
This is the difference between the magnetic and geographic poles
Magnetic declination
This is a hypothetical surface of Earth
Geoid
Is a geoid smoother than a reference ellipsoid?
No
Is a geoid smoother than Earth’s surface?
Yes
A geoid’s shape is a result of variation in this
Earth’s gravity
A geoid coincides with this if the oceans were extended through the continents
Mean sea level
For a geoid shape, this is measured for many ground points
Surface gravity
For a geoid shape, these are interpolated using mathematical models
Unmeasured areas
These tools were initially used for measuring/interpolating geoid shape
Plumb bobs
What are three modern tools used to measure geoid shape now?
Ground gravimeters, airplanes and satellites
On a planetary geoid, surface elevation is usually defined as this
Orthometric height
This is the difference between a geoid and an ellipsoid
Geoidal undulation
This explains why different ellipsoids are used in different regions
Geoidal undulation
These form basis for calculating the position of points on Earth
Geodetic datums
What is the geodetic agency for the United States?
National Geodetic Survey
What are the two types of geodetic datums?
Horizontal and vertical datums
These datums define horizontal position with lat-long or other coordinates
Horizontal datums
What are the two components of horizontal datums?
- Reference ellipsoid and coordinate system; 2. Many precise ground points
This component of a horizontal datum comprises the theoretical model
Reference ellipsoid/coordinate system
This component of a horizontal datum fixes a model to the surface
Many precise ground points
This is a network of precise ground points
Geodetic survey
Historic geodetic surveys use these to precisely measure a few control points, then triangulating off of them
Stars
Modern geodetic surveys use these
Satellites
This process uses trigonometry to position points in geodetic surveys
Triangulation
Between what years did the Great Trigonometric Survey of India take place?
1802-1866
These are monumented datum points
Benchmarks
What are the three common geodetic datums used in the U.S.?
- North American Datum of 1927 (NAD27); 2. North American Datum of 1983 (NAD83); 3. World Geodetic System of 1984 (WGS84)
Who performed the North American Datum of 1927?
U.S. Coast and Geodetic Survey
Which ellipsoid did the North American Datum of 1927 use as a reference?
Clark 1886 ellipsoid
What did the North American Datum of 1927 use as a base point?
Meades Ranch, KS
How many survey stations were involved in the North American Datum of 1927?
26,000
Who performed the North American Datum of 1983?
National Geodetic Survey
What did the North American Datum of 1983 use as references?
GRS80 ellipsoid and geocentric origin
How many survey stations were involved in the North American Datum of 1983?
250,000
Who performed the World Geodetic System of 1984?
Department of Defense
What did the World Geodetic System of 1984 use as references?
WGS84 ellipsoid and geocentric origin
This results in benchmarks having different lat-long with different datums
Datum shift
This uses math conversion to estimate datum shift
Datum transformation
Between what two datums have the largest datum shifts occurred?
NAD27 and newer datums
Are shifts with modern datums usually large?
No
Is transforming NAD27 to modern datums complicated?
Yes
Is transforming between modern datums complicated?
No
Do NAD83 and WGS84 have their own transformations?
Yes
Should GIS projects mix datums?
No
Should caution be used when transforming datums?
Yes
This type of datum is used for defining surface elevations
Vertical datums
Vertical datums use a network of these
Established ground points
Vertical datums include precise measurements of these two things
Elevation and gravity
Do vertical datums share many stations with horizontal datums?
Yes
Vertical datums used these historically
Leveling surveys
Where do vertical datums start and end?
Ocean to inland
In modern vertical datums, what are surface elevations estimated with?
Satellites
What are two common North American datums?
National Geodetic Vertical Datum of 1929 (NGVD29) and North American Vertical Datum of 1988 (NAVD88)
Leveling surveys use this to estimate height
Trigonometry
These are flat representations of the Earth’s surface
Map projections
Map projections are easier to use/analyze than these representations of the Earth
Globes
What are three of five key properties of maps?
Scale, shape, area, distance, direction
Map projections are a systematic transfer of these to a flat surface
Points on datum
Will all map projections distort the Earth?
Yes
Do map projections distort all map properties?
No
Do map projections preserve some properties and distort others?
Yes
Can distortion change within a single map?
Yes
Where is distortion lowest on a map?
Near intersection of ellipsoid and map
Do all maps have a point or line where distortion is zero?
Yes
The shortest path on a globe is represented by this on a map
Arc
What are the three parameters of projections?
- Projection surface; 2. Projection perspective; 3. Projection aspect
What are three common projection surfaces/shapes?
Cylinder, cone, plane
What are three common map projection perspectives?
Gnomonic, stereographic, orthographic
What are three map projection aspects?
Normal, transverse, oblique
What are two map projections used in GIS?
Lambert Conformal Conic and Transverse Mercator
This map projection uses a conical surface
Lambert Conformal Conic
These are the two lines of latitude that intersect the ellipsoid in the Lambert Conformal Conic map projection
Standard parallels
How much distortion do standard parallels have in a Lambert Conformal Conic map projection?
Zero
What can be done to control distortion on a Lambert Conformal Conic map projection?
Change position of parallels
What direction does the low distortion band run on a Lambert Conformal Conic map projection?
East-West
The Lambert Conformal Conic map projection is a good projection for this type of region
Wide regions
This map projection uses a cylindrical surface
Transverse Mercator
In what places does the Transverse Mercator map projection intersect the ellipsoid?
Along one tangent or two secant lines
Where is distortion lowest on a Transverse Mercator map projection?
Near intersection
In a Transverse Mercator map projection, what can be changed to control distortion?
Secant positions
This is the line of longitude between secants in a Transverse Mercator map projection
Central meridian
Which way does the low distortion band run in a Transverse Mercator map projection?
North-South
The Transverse Mercator map projection is a good projection for this type of region
Tall regions
This projection system is used by U.S. states
State Plane System
What are three fields that use the State Plane System?
Surveying, construction, GIS
The State Plane System converts this to an x-y coordinate system
Lat-long
In the State Plane System, does each state use its own coordinates and origin?
Yes
In the State Plane System, states are zonated to reduce this
Distortion
How many zones can a state have in the State Plane System?
1-6
In the State Plane System, to what maximum is distortion kept?
1 part in 10,000
Do all states use the same map projection type in the State Plane System?
No
What map projection do tall states use?
Transverse Mercator
What map projection do wide states use?
Lambert Conic
Can a state use both map projections for different zones?
Yes
The State Plane System was first developed for this datum
NAD27
This datum introduced some changes to the State Plane System
NAD83
This is a global position coordinate system
Universal Transverse Mercator System
Into how many zones does the Universal Transverse Mercator System split the globe?
60 zones
How wide is each global zone in the Universal Transverse Mercator System?
6 degrees
Between what two boundaries does the Universal Transverse Mercator System extend?
From 80 degrees South to 84 degrees North
How much overlap is there between zones in the Universal Transverse Mercator System?
40 km
In the Universal Transverse Mercator System, each zone is split into what two zones?
North and south
What does the Universal Transverse Mercator System use in its coordinate system?
Meters from origin
In the Universal Transverse Mercator System, is the cylinder ellipsoid smaller or larger than Earth’s diameter?
Smaller
Can spatial data use different map projections when working in GIS?
No
Can map projections be converted?
Yes
GIS software uses these to transform coordinates
Math equations
Complex transformations can include both these types of conversions
Datum and map projection conversions
In converting map projections without datum conversion, map projection coordinates must be first back-converted to these
Geographic coordinates
Is converting map projections with different datums simple to do?
No
Converting map projections with datum conversion requires this step
Datum conversion step
This is a U.S. land parcel coordinate system
Public Land Survey System
The original U.S. colonies used this land parcel system
Metes and bounds
The metes and bounds system uses these to bind parcels
Geography and local markers
In what year was the Public Land Survey System established by Congress?
1785
What are the two basic Public Land Survey System units of area?
Township and section
What are the dimensions of a township in the Public Land Survey System?
6x6 mile parcel
Into how many sections is a township divided in the Public Land Survey System?
36
In the Public Land Survey System, townships are located with these two coordinates
Township and range coordinates
What are the dimensions of a section in the Public Land Survey System?
1x1 mile parcel
Into what is a section divided in the Public Land Survey System?
Quarters and small parcels
In the Public Land Survey System, township and range coordinates use these two types of lines
Principal meridians and baselines
