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Geog 480 > Topic 9 > Flashcards

Topic 9 Flashcards

1
Q

how do you determine what is the best coordinate system

A

depends on what you are doing/mapping

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2
Q

describe the projection process

A

choose how to model the earth
principale scale = scale reduction
reduced to the globe

global = sphere
small = ellipsoid

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3
Q

what does GCS do

A

tell me where on the earth the data should draw

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4
Q

what does the PCS do

A

tell me how to draw on the earth on a flat surface

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5
Q

GCS

A

latitude and longitude
3d reference type on a spherical surface

angular measures (degrees, minutes, seconds)

N,E are +
S,W are -

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6
Q

the geoid

A

equipotential surface
surface rises under continents and lowers under oceans - generally
variations in gravitational force due to irregular distribution of rocks

earth shaped

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7
Q

the ellipsoid

A

a regular geometric surface used to approximate the geoid

oblate or “flattened”

has easily defined axes

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8
Q

how to choose ellipsoid?

A

before 1980s different ellipsoids used for different areas.

satellite data allowed to choose a best fitting ellipsoid for the whole earth
(WGS84)

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9
Q

datums and characteristics

A

shape of ellipsoid

orientation of ellipsoid

size, position, orientation (3 key points)

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10
Q

what are the two types of datums

A

ECEF ( earth fixed) global
WGS84

Not ECEF (not earth fixed) local
NAD27
NAD83

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11
Q

prominent horizontal datums

A

NAD27

NAD83

WGS84

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12
Q

NAD27

A

north american datum 1927

based on clarke 1866 ellipsoid

NOT ECEF

basis for horizontal datum

developed to map North America

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13
Q

NAD83

A

north american datum 1983

based on GRS1980 ellipsoid

less flattened than NAD27

PLATE FIXED

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14
Q

WGS84

A

world geodetic system

slight change from the NAD 83 (2m + difference only)

slight orientation change

based on GRS1980

ECEF

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15
Q

ECEF vs. plate fixed

A

ECEF = earth centered, earth fixed
location is same, but earth might have moved

plate fixed = move lat and long locally according to earth movement

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16
Q

what is an epoch

A

mathchig timeframe of measurements to our maps
timeframe when the snapshot was taken

epoch = realization

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17
Q

what is the ITRS

A

international terrestrial reference system

describes procedures for creating reference frames

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18
Q

what is the ITRF

A

international terrestrial reference frames

realization of the ITRS
new reference frame solutions produced
snapshot of coordinates
lat/long elevation at point in time

updated every few years

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19
Q

what does a G730 realization mean

A

= # of weeks since Jan 1st 1980

G=GPS

so 730 weeks since Jan 1st 1980

20
Q

What is a GCS?

A

main components
sperhical coordinate system
mathmatical model (datum)

21
Q

what is EPSG

A

european petroleum survey group

stnadards for projections and coordinates

22
Q

vertical datums

A

defines 0 altitude, so references sea level
before 1980s based on measurments of ‘mean sea level”

NAVD88

CGVD2013

GNSS (based on ellipsoid not sea level measurments)!!

23
Q

what do vertical datums show

A

shows difference in height between ellipsoid and geoid

H = orthometric height
N = geoid height
h = ellipsoid height

H = h - N

if this equals -# that means ellipsoid is above the geoid

24
Q

what is CSRS

A

canadian spatial reference system

collection of standards, models, data products and infrastructure supportin geospatial positioning

coordinate base is also a reference frame like ITRF (canadian is plate fixed)

25
Q

if your map is off mostly likely someone has incorrectly defined the reference system
T/F?

A

true

70m is a small error for canada as a whole

erros are inherently built inot the NAD27

26
Q

what reference system is used for Canada

A

NTv2
NADCOW for USA

very small difference between the NAD83 and the updated 2010 Alberta model of NAD83

27
Q

reference difference in alberta

A

last arc second = aprx. 30 meters
long arc second = depends what lat

at north pole + = so tiny
much larger at equator

28
Q

PCS

A

cartesian
how ti should be drawn on a flat surface
drawing a 3d volume on a flat surface
distortions will happen (golden triangle problem)
choice of projection depends on what you are doing
descringly important as cartogprahy scale increases (zoom in)

bigger area = projection more important
converting to x,y point positions

29
Q

what are map projections families based on

A

developable surfaces (“the paper”)
plane, cone, cyclinder

30
Q

Azimuthal projections

A

gnomic projection
source at centre of earth
used for navigation charts

orthographic
infinite light source
looks like earth from space

stereographic
light from back or side of globe
common historically
mapping polar regions

lambert conformal basemap for BC (used heaviley in Canada)

31
Q

what is the basemap for alberta

A

tranverse mercator
canada = lambert conformal conic

32
Q

what are the 2 common cylindrical projections

A

mercator :wrapped around equator

transverse mercator : wrapped around poles

33
Q

4 key properties for projection distortion

A

shape
area
distance
direction

each projection is designed to achieve one or two of these

34
Q

what is tissot’s indicatrix

A

unit circle on globe
visual help in understanding distortions

preserve shape = conformal
angles are the same
size will vary

preserve area = equal area
area preserved
angles compromised

35
Q

shape : conformal projections

A

scale is the same in every direction around any given point (isotropic)
scale changes between areas of the map
shapes for small regions are portrayed accurately

cant compare sizes

lambert conformal conic

36
Q

area: equal are projection

A

scale in small area changes depending on direction (anisotropic)
shapes in small areas are distorted
cant compare shape

bonne projection
albers equal area vs lambert conformal

37
Q

distance: equidistant projections

A

true distances
distance between any point and the north pole is true
doesnt work for any two random points tho
usualyl azimuthal

no map can be conformal and equal area at the same time

azimuthal equidistant projection
conic equidistant projection

38
Q

direction

A

direction measurements on the map same as those on the ground
can be conformal, mercator
can be maintained simultaneously with one or two of the other 3 properties

39
Q

projection surface: tangent

A

stretching: where paper is above the globe
c is not distorted
a and e are stretched a lot
b and d are stretched a little

1 standard line

40
Q

projection surface: secant

A

shrinking: there paper is “inside” the globe
b and d are not distorted
a and e stretched (paper is above the globe)
c is shrunken

2 standard lines

41
Q

patterns of distortion (aimuthal)

A

tangent = 1 standard line
secant = 2 standard lines

42
Q

how does projection and concept of PCS work

A

plotting cartesian points on a x,y graph

43
Q

universal transverse mercator (UTM)

A

basis of many topographic maps
globe is divided into 60, 6 degree longitudinal strips

each strip is “numbered zone”

subdivided into latititude strips

44
Q

transverse mercator characteristics

A

cyclindrical conformal projection
secant
pick meridian
6 degrees of longitude or strips, or zones
maps are windows in the zones
doesnt go all the way to the polar regions but only 84 degrees north and 80 degrees south

central meridian moves with the projection

UTM is only unique to a geographic coordinate system
standard lines are not actually defined
center of meridian = 500,000 m E
so that we are always using positive numbers

45
Q

is NAD83 CSRS a GCS?

A

yes

46
Q

definition of a map projection

A

orderly system of parallels and meridians used to draw a map on a flat surface

distortion must occur when transforming information from 3d to a flat surface

choice of projection is dependent on the purpose of the map

decreasingly important as cartographic scale increases

47
Q

choice of map description

A

go by desired properties
basic guidelines
there is no right answer only good and bad choices
trying to match the projection with the shape of the mapped earth
plate fixed or ECEF

Geog 480 (14 decks)

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