Lecture 4 Understanding Coordinate Systems
Lecture 4 Understanding Coordinate Systems. Geographic Coordinate systems GCS Spherical Ellipsoidal Curved.
Dec 25, 2015
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Lecture 4
Understanding Coordinate Systems
Geographic Coordinate systems
GCS
Spherical Ellipsoidal Curved
Projected coordinate systems
GCS
PCS
(PCS) 2D Flat Planar Cartesian
GCS has angular units of measure Degrees
360 per circle Decimal degrees Degree, minute, second
Radians 2 pi per circle ~6.3 per circle (~57 degrees each)
Gradian 400 per circle
Gon Same as gradians To some grad = degree
X -Y +
X +Y +
X -Y -
X +Y -
X
Data
usually here
Y
PCS has linear units of measure Linear units
Meters Feet
X and Y coordinates Length, angles, and areas are constant
Map projection Math to transform GCS
Map projection
Plate Carrée projection
Math to transform GCS to PCS Flattening the earth – round to flat Distortions make geographers SADD
Shape, Area, Distance, and Direction
PCS properties example
Name – NAD 1983 UTM Zone 11N GCS – NAD 1983 Map Projection – Mercator Projection parameters
Central meridian, latitude of origin, scale factor, false easting
Linear unit of measure (i.e., meters)
Geographic coordinate systems Mathematical model of a planetary body - spheroid Parameters describe the spheroid shape
Smooth, without imperfections GCS for earth, planets, and more
Earth Mars IO
GCS properties Spheroid
Major and minor axis Units (lat/long, radians, grads)
GCS properties Spheroid
Major and minor axis Units (lat/long, radians, grads)
Datum Spheroid’s position in relation to actual earth Local datum: spheroid touches edge of earth, good fit there
Great fit here
Bad fit here
Local datum
GCS properties Spheroid
Major and minor axis Units (lat/long, radians, grads)
Datum Spheroid’s position in relation to actual earth Local datum: spheroid touches edge of earth, good fit there Earth-centered: spheroid and earth center match
Great fit here
Bad fit here
Local datum
All around best fit for the entire planet
Earth-centered datum
GCS properties example Name
European Datum 1950 Datum
European Datum 1950 Spheroid
International 1924
Prime Meridian Greenwich
Angular unit of measure Degrees
GCS with a local datum Spheroid
GCS with a local datum Datum
Spheroid’s position in relation to actual earth
GCS with a local datum Datum
Spheroid’s position in relation to actual earth
GCS with a local datum Datum
Spheroid’s position in relation to actual earth
GCS with a local datum Datum
Spheroid’s position in relation to actual earth
GCS with a local datum Datum
Spheroid’s position in relation to actual earth
GCS with a local datum Datum
Spheroid’s position in relation to actual earth
Local datum: spheroid touches edge of earth, good fit there
Bad fit on the other side
GCS with an Earth Centered datum Spheroid
GCS with an Earth Centered datum Datum
Spheroid’s center matched to earth center
GCS with an Earth Centered datum Datum
Spheroid’s center matched to earth center
GCS with an Earth Centered datum Datum
Spheroid’s center matched to earth center
GCS with an Earth Centered datum Datum
Spheroid’s center matched to earth center
GCS with an Earth Centered datum Datum
Spheroid’s center matched to earth center
GCS with an Earth Centered datum Datum
Spheroid’s center matched to earth center
Best fit all around the earth
As measurement gets better, new GCS are defined NAD27 – parameters defined in 1866 (log tables) NAD83 – parameters defined in 1979 (pre-GPS) WGS84 – parameters defined in 1984 (GPS)
Common GCS parameters in use today (US)
North American Datum 1927
North American Datum 1983
World Geodetic Survey 1984
Warning: different geographic coordinate system…
Geographic transformation Math to transform from one GCS to another
NAD 27
34 Degrees
3 Minutes
23.1 Seconds
North
ESRI-Redlands
117 Degrees
11 Minutes
39.2 Seconds
West
Geographic transformation Math to transform from one GCS to another Changing GCS changes the lat/long for same point The same spot on earth has differing coordinates
NAD 27NAD 83
34 Degrees
3 Minutes
23.14 Seconds
North
ESRI-Redlands
117 Degrees
11 Minutes
42.36 Seconds
West
34 Degrees
3 Minutes
23.1 Seconds
North
ESRI-Redlands
117 Degrees
11 Minutes
39.2 Seconds
West
ArcMap’s GCS and PCS behavior Data frame - has both Spatial data - has GCS, may have PCS Metadata - prj, XML, mdb, or none Tools that help
On-the-fly projection
ArcMap data frames have a GCS and a PCS You should set them If not set, data frames take first layer’s GCS/PCS
Data frame: Bonne PCS
On-the-fly projection ArcMap data frames have a GCS and a PCS
You should set them If not set, data frames take first layer’s GCS/PCS
If needed, new layers are projected on-the-fly (to match) If no CS metadata, new layer cannot be projected on-
the-fly
Input layer: Robinson PCS Data frame: Bonne PCS
ArcMap projects data on-the-fly into a data frame
GCS and PCS metadata for spatial data Stored in internal geodatabase tables Stored in projection files
Shapefiles can have a .prj text file (e.g., streets.prj) Coverages can have a prj.adf text file (e.g.,
/rivers/prj.adf) Stored optionally in XML files created by ArcCatalog Non-native ESRI datasets use various other formats
Warning!
GCS and PCS metadata is NOT required You might get data that is missing its
coordinate system metadata If researched and discovered, you can add it If not, use Spatial Adjustment to move the
data into place
Spatial reference problems and solutionsProblem Solution
You know the coordinate system information, but it is missing
Define Projection tool
The PCS is defined correctly,
but is not the one you need
Project tool or data frame project on-the-fly
The GCS is defined, but it is not NAD27 or NAD83
Project tool or set a geographic
transformation in the data frame
properties
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