Projections

Question 1

What is the basic concept behind projections?

Answer 1

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)

Question 2

What is equivalence?

Answer 2

equal area property assuring that the relative sizes of all map features are correct. It is possible to make equivalent maps.

Question 3

What is conformality?

Answer 3

angular deformation property assuring that local angles are not distorted, so shapes are generally correct

Question 4

What is azimuthality

Answer 4

property that straight lines on the map represent the shortest paths on the sphere. Referred to as “true direction.”

Question 5

what is equidistance?

Answer 5

property that the scale of the map is correct so distances are measured correctly.

Question 6

Can a map projection provide every earth property at once?

Answer 6

No. For instance, Equivalence and conformality are incompatible

Question 7

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 _______.

Answer 7

conformal

equivalent

Question 8

What are examples of indicatrix on maps?

Answer 8

Looks at slide 9 in “Projections”

Question 9

What is the importance of Equivalence?

Answer 9

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.

Question 10

What is the most popular US Equivalent Projection?

Answer 10

Albers equal area

Question 11

What is the importance of conformality?

Answer 11

We use conformal maps to make standard Cartesian grids that must maintain right angles throughout.

Look at slide 14 for example of image.

Question 12

What is the most popular US Comformal Projection?

Answer 12

Lambert Conformal Conic

Question 13

What is the importance of Azimuthality?

Answer 13

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.

Question 14

What is the importance of Equidistance?

Answer 14

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

Question 15

What is a gnomonic projection?

Answer 15

An azimuthal projection - all straight lines are great circles.

Look at slide 19

Question 16

Parallels are ______
Parallels get ____ toward the poles as on a globe (as we saw in Excel)
Meridians are all the _______
Meridians converge toward ______

Answer 16

parallel
shorter
same length.
the poles

Question 17

Areas formed by the graticule lines are:

Answer 17

equal at equal latitude

smaller toward the poles

Question 18

If the parallels and meridians do not cross at right angles, then it is not ______.

Answer 18

Conformal

Question 19

What two generalizations can we make about projections?

Answer 19

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.

Question 20

What are standard lines? What’s special about these lines?

Answer 20

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.

Question 21

Is an equivalent projection equivalent everywhere?

Answer 21

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!

Question 22

What are cartesian coordinates?

Answer 22

The results of projection from geographic coordinates

Question 23

What are the standard parameters for projections?

Answer 23

Standard parallel
Central Meridian
Latitude of Origin 
False Easting
False Northing

Question 24

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.

Answer 24

One of two conformal projections

Standardized and stored in projection software (like GIS)

Question 25

What is a Lambert Conformal Conic?

Answer 25

Slide 29

Question 26

What is a Transverse Mercator?

Answer 26

Slide 30

Question 27

State Plane Coordinates (SPC) provide three things? What are they?

Answer 27

1) A zone or more for each state as needed to meet the error standard required.
2) Less than .01% error between projection on which it is based and surveys of the corresponding earth-based points. This requirement effectively limits how wide a “zone” can extend out from the standard lines (about 158 miles on the Lambert Conformal Conic).
3) Conformal projection choice (of 2) is based mainly on a state’s shape.

Question 28

What is the Universal Transverse Mercator?

Answer 28

This grid system uses 60 map projections to cover the entire world from 80 S to 84 N. Each zone is 6  in width and covers 164  of latitude. This map projection system gives us 0.04% or better accuracy in point placement.

Slide 34 for image

Question 29

In GIS we need to use projections that _________.

Answer 29

Fit the area we are mapping and analyzing. Often we have a choice, and even those choices do not completely agree. If you pick a bad projection, it can create very inaccurate data.