Projections Flashcards
What is the basic concept behind projections?
Take 3D data measured in angular locations called latitude and longitude, add some localization parameters, and translate them to 2D Cartesian coordinates called eastings and northings (and vice-versa)
What is equivalence?
equal area property assuring that the relative sizes of all map features are correct. It is possible to make equivalent maps.
What is conformality?
angular deformation property assuring that local angles are not distorted, so shapes are generally correct
What is azimuthality
property that straight lines on the map represent the shortest paths on the sphere. Referred to as “true direction.”
what is equidistance?
property that the scale of the map is correct so distances are measured correctly.
Can a map projection provide every earth property at once?
No. For instance, Equivalence and conformality are incompatible
A simple way to look for equivalence and conformality is through a set of small circles over the graticule crossings on Earth.
If all of the circles remain circles (correct shape) on the projection, the projection is _______.
If the circles become ellipses on the projection, but their areas are the same throughout, the projection is _______.
conformal
equivalent
What are examples of indicatrix on maps?
Looks at slide 9 in “Projections”
What is the importance of Equivalence?
Some projections greatly distort areas. If you need to compute areas of continents, countries, etc, you must have an equivalent projection.
Look at slide 12 for example of image.
What is the most popular US Equivalent Projection?
Albers equal area
What is the importance of conformality?
We use conformal maps to make standard Cartesian grids that must maintain right angles throughout.
Look at slide 14 for example of image.
What is the most popular US Comformal Projection?
Lambert Conformal Conic
What is the importance of Azimuthality?
Some projections greatly distort direction, so the apparent route that you would follow to travel from one place to another is not the actual best route.
While straight lines on a Mercator are constant compass bearings and will get you to your destination, they are not the shortest route.
Look at slide 16 for image.
What is the importance of Equidistance?
Some projections greatly distort distance so a straight line on the map does not compute the correct distance when converted by the map’s scale (soon).
Look at slide 17
What is a gnomonic projection?
An azimuthal projection - all straight lines are great circles.
Look at slide 19
Parallels are ______
Parallels get ____ toward the poles as on a globe (as we saw in Excel)
Meridians are all the _______
Meridians converge toward ______
parallel
shorter
same length.
the poles
Areas formed by the graticule lines are:
equal at equal latitude
smaller toward the poles
If the parallels and meridians do not cross at right angles, then it is not ______.
Conformal
What two generalizations can we make about projections?
1) Flattening larger areas removes more curvature, is more challenging and more error occurs.Overall, regardless of the projection properties, smaller areas are projected more accurately.
2) Areas nearest a projection’s standard line(s) are more accurate than those farther from the standard lines as the standard lines are always the correct scale.
What are standard lines? What’s special about these lines?
Locations where the globe and the map surface intersect or have tangency. They are the correct length on the map, all other lines are typically not correct. We get to choose their location, so we put them near the area to be mapped.
Is an equivalent projection equivalent everywhere?
yes, while we can maintain accurate area, shape etc. across a map, we cannot maintain the opposite property. SO, well placed standard lines make the shapes around them better even if we maintain area as the property, and vice versa!
What are cartesian coordinates?
The results of projection from geographic coordinates
What are the standard parameters for projections?
Standard parallel Central Meridian Latitude of Origin False Easting False Northing
Major national systems and international systems are based on ___________depending upon the relative areal extent of the mapped area.
Parameters for the projection “black box” are _________ to create these grids.
One of two conformal projections
Standardized and stored in projection software (like GIS)
