Topic 9 Flashcards
how do you determine what is the best coordinate system
depends on what you are doing/mapping
describe the projection process
choose how to model the earth
principale scale = scale reduction
reduced to the globe
global = sphere
small = ellipsoid
what does GCS do
tell me where on the earth the data should draw
what does the PCS do
tell me how to draw on the earth on a flat surface
GCS
latitude and longitude
3d reference type on a spherical surface
angular measures (degrees, minutes, seconds)
N,E are +
S,W are -
the geoid
equipotential surface
surface rises under continents and lowers under oceans - generally
variations in gravitational force due to irregular distribution of rocks
earth shaped
the ellipsoid
a regular geometric surface used to approximate the geoid
oblate or “flattened”
has easily defined axes
how to choose ellipsoid?
before 1980s different ellipsoids used for different areas.
satellite data allowed to choose a best fitting ellipsoid for the whole earth
(WGS84)
datums and characteristics
shape of ellipsoid
orientation of ellipsoid
size, position, orientation (3 key points)
what are the two types of datums
ECEF ( earth fixed) global
WGS84
Not ECEF (not earth fixed) local
NAD27
NAD83
prominent horizontal datums
NAD27
NAD83
WGS84
NAD27
north american datum 1927
based on clarke 1866 ellipsoid
NOT ECEF
basis for horizontal datum
developed to map North America
NAD83
north american datum 1983
based on GRS1980 ellipsoid
less flattened than NAD27
PLATE FIXED
WGS84
world geodetic system
slight change from the NAD 83 (2m + difference only)
slight orientation change
based on GRS1980
ECEF
ECEF vs. plate fixed
ECEF = earth centered, earth fixed
location is same, but earth might have moved
plate fixed = move lat and long locally according to earth movement
what is an epoch
mathchig timeframe of measurements to our maps
timeframe when the snapshot was taken
epoch = realization
what is the ITRS
international terrestrial reference system
describes procedures for creating reference frames
what is the ITRF
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
what does a G730 realization mean
= # of weeks since Jan 1st 1980
G=GPS
so 730 weeks since Jan 1st 1980
What is a GCS?
main components
sperhical coordinate system
mathmatical model (datum)
what is EPSG
european petroleum survey group
stnadards for projections and coordinates
vertical datums
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)!!
what do vertical datums show
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
what is CSRS
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)
if your map is off mostly likely someone has incorrectly defined the reference system
T/F?
true
70m is a small error for canada as a whole
erros are inherently built inot the NAD27
what reference system is used for Canada
NTv2
NADCOW for USA
very small difference between the NAD83 and the updated 2010 Alberta model of NAD83
reference difference in alberta
last arc second = aprx. 30 meters
long arc second = depends what lat
at north pole + = so tiny
much larger at equator
PCS
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
what are map projections families based on
developable surfaces (“the paper”)
plane, cone, cyclinder
Azimuthal projections
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)
what is the basemap for alberta
tranverse mercator
canada = lambert conformal conic
what are the 2 common cylindrical projections
mercator :wrapped around equator
transverse mercator : wrapped around poles
4 key properties for projection distortion
shape
area
distance
direction
each projection is designed to achieve one or two of these
what is tissot’s indicatrix
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
shape : conformal projections
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
area: equal are projection
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
distance: equidistant projections
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
direction
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
projection surface: tangent
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
projection surface: secant
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
patterns of distortion (aimuthal)
tangent = 1 standard line
secant = 2 standard lines
how does projection and concept of PCS work
plotting cartesian points on a x,y graph
universal transverse mercator (UTM)
basis of many topographic maps
globe is divided into 60, 6 degree longitudinal strips
each strip is “numbered zone”
subdivided into latititude strips
transverse mercator characteristics
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
is NAD83 CSRS a GCS?
yes
definition of a map projection
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
choice of map description
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