Coordinate Systems Flashcards
What is the difference between a projected and an unprojected coordinate system?
unprojected: units in decimal degrees
projected: units in meters (or feet)
-projecting changes the x-y values from degrees to meters or feet
Describe in detail the GCS, or the Geographic Coordinate System.
A 3-dimensional coordinate system that locates points on earth’s surface. measured in angular degrees. based on spherical global coordinates. degrees of latitude and longitude, not meant to be shown in 2D.
What is the datum? Describe it in terms of the ellipsoid.
earth is not a perfect sphere, but the geoid is too irregular to map so we use the ellipsoid as a next-best fit. The datum shifts the ellipsoid relative to the geoid to achieve a best fit between the two.
a datum definition includes: the particular ellipsoid (major and minor axis) chose AND the adjustment or fit (translation of center, to minimize distortion)
What is the difference between a local and a geocentric datum?
-local datum: optimizes the shift for the best fit at a particular location
-geocentric or world-centered datum: optimizes the fit for the entire earth
What are the 3 main types of map projections?
conic
azimuthal
cylindrical
be able to describe the central meridian, reference latitude, false easting and northing, and tangent vs. secant projections
central meridian: the longitude which serves as the x=0 origin of the map
reference latitude: latitude that serves as the y=0 origin for the map
false easting and northing: arbitrary values added to x and y values. usually to ensure that all x-y coordinates are positive
tangent projection: has single standard parallel
secant projection: has two standard parallels
what map properties are preserved and which are distorted for each of the 3 main projection types?
cylindrical: preserves direction and shape / distorts distance and area
conic: preserves area and distance / distorts shape and direction
azimuthal: preserves area and distance / distorts shape and direction
when might each projection type be useful, depending on your map purpose?
cylindrical projections when preserving direction and shape are important
conic or azimuthal equidistant projections when distances must be correct
conic or azimuthal equal area projections when areas must be correct
conformal or compromise projections for general purpose maps
for which projections can you NOT use a compass rose?
what is the difference between the “project” tool and the “define projection” tool? What is a “transformation”?
Define Projection only modifies the LABEL of the coordinate system (use only when CS is unknown or incorrect)
Project creates a new dataset in a different coordinate system (use when changing a CS permanently/ use to assemble collections of data with the same stored CS)
what would happen if you changed the projection in the catalogue window? what are the ways in which you can properly change a projection without altering the original coordinate system stored with the file?
It would only alter the label of the coordinate system. We can change a projection without altering the original CS by letting Arc project on-the-fly
how do you georeference a raster? what purpose might this serve?
a raster that already has the coordinate system information ready for use is said to be georeferenced
-if the coordinate system is unknown, use “ground control points” to match locations in the image to locations with a known CS
how do you convert from arc seconds to degrees?
1 arc minute = 1/60th of a degree
1 arc second = 1/60th of an arc minute
how do you project rasters?
Cell centers are converted to the new system. New cell size must be specified. Resampling must occur
how do you properly format an excel file to make sure that it will upload in Arc?
First row must contain field headings with legal field names. No blank rows or formulas should be used. Each column should contain only text or only numbers. Format the cells. Export as .xls.
