Map and Chart Projection – Ibex Aviation Academy

Projections

Projections are used to make maps and charts by projecting spherical (3D) earth on a paper (2D)
Traditionally, map preparation was done in three steps
Step 1 – A 3D transparent model of earth with latitudes and longitudes called reduced earth is made to scale
Step 2 – Light is projected from inside the reduced earth on a wrapped photographic paper
Step 3 – The wrapped paper is then cut opened to show latitudes and longitude
Perspective projections are obtained by projecting light and are used without any mathematical modifications
Perspective projections are mathematically modified to get non-perspective projections before usage

An Ideal chart possesses eight properties of which some are important for aviation
The chart should be Orthomorphic or Conformal
Measurement of angles and bearings must be accurate
Angles measured on a chart must be same as on earth surface
Scale must be correct as well as constant throughout the chart
Shapes of features must be the same as that of earth
Equal areas on earth must be shown as equal areas on chart
Location at different latitudes must not change the size of a place
Rhumb lines as well as Great circles must be straight lines
Latitudes and longitude must be straight lines and cut at right angles
It should be easy to plot on a map or chart
Adjacent sheets should fit correctly without any gap or overlap
One projection should be able to cover through out the world

Orthomorphism or Conformality means that the angles and bearings measured on the chart must be same as that on earth
Aviation charts must be orthomorphic to draw and fly a specific track
A chart can be orthomorphic two conditions are satisfied
Meridians and parallels must cut at right angles
Scale must be the same in all the directions around a point
Alternately, scale expansion must be constant in all the directions
Scale expansion in North South as well as East West must be equal

Correct Scale means that the distance measured using the given scale must be same as that of earth
If scale is 1 Cm = 20 NM, it should be correct throughout the projection
Correct scale is possible if light travels the same distance in reduced earth as well as that of paper
At the Parallel of tangency or origin, the scale would be correct since light travels the same distance
Scale would be incorrect in other latitudes, since light would travel more or less distance
Scale error is the difference between the scale mentioned in the chart and the scale at any point on the chart
ICAO has accepted up to a scale error of up to 1% as correct scale chart

Ideally, scale must be correct and constant throughout the chart
Constructing a constant scale chart would result in incorrect scale, since earth distance between meridians reduces at higher latitudes
In short, scale cannot be constant as well as correct in any projection
Correct Scale charts are preferred over constant scale charts for aviation purposes

Ideally, shapes of features must be the same as that of earth
This property can be met only if the latitudes and longitudes are curved lines as that of earth
Curved latitudes and longitudes are not usable for plotting charts
Distortion in shapes does not affect chart usage and are therefore acceptable for aviation charts

Ideally, equal areas on earth must be shown as equal areas on chart
Constant scale chart would maintain equal areas but we had decided that correct scale is more important than constant scale
Expansion or contraction in areas does not affect air navigation and are therefore acceptable for aviation charts

Ideally, Great circles and rhumb lines must be straight lines
On the earth, great circles are straight lines while rhumb lines are uniformly curved lines
Therefore, great circles and rhumb lines cannot be straight lines on any chart
Either rhumb lines or great circles can be made straight which would automatically convert the other one to curved line

Ideally, latitudes and longitudes must be easy to plot
Parallels and meridians must be straight lines and must intersect at right angles for ease of plotting
Aviation charts used for plotting purposes must meet this criteria

Ideally, adjacent sheets should fit properly without gaps or overlap
No gap should be seen when charts are kept close to each other but a slight overlap can be accepted

Although an Ideal Chart has eight properties three conditions are essential for aviation use
Orthomorphism and Correct scale are most important for pilots
Ease of plotting is also important for aviation purposes
Either Great circles or Rhumb Lines should be straight lines