Getting grounded in GIS: Map making

GIS starts with mapping.  Map making is cartography.  The earliest known map is dated 6,200 BC: a Babylonian clay tablet recovered in 1930 that depicted district boundaries, hills and water features.  Mapping helped to advance human knowledge because it was a new way to capture and share information with each other and future generations.  Passing information to future generations is a basis for developing culture.

Maps of the stars are also maps.  Caves with dots on the walls that represent stars have been found as old as 16,500 BC.  There are other drawing of caves older than the 6,200 BC tablet that represent mountains and rivers.  It is hard to determine if these older drawings of Earth were actual geographic features or just drawings.

I often joke with GIS professionals that they just make pretty maps.  In reality, GIS professionals take a large three-dimensional object (known as the Earth), merge it with data sets through geo-coding, and then provide it to customers in an easy to understand two-dimensional image.  Computers that run GIS are often the most powerful systems in an office because of the large data sets, complex graphics, and significant calculations to perform.

Mapping’s inherent challenge has always been to take a three dimensional object and project it in two dimensions.  A map can provide information on area, shape, direction, bearing, distance and scale.  However, every projection distorts some properties to keep others accurate.  There is no two-dimensional projection that accurately captures all properties of a three-dimensional object.  It is important to select the best projection for the information that is needed.

How hard is it to convert a 3D object to a 2D representation?  Next time you are peeling an orange, try to get the peel off in a few large sections.  Flatten the orange peel.  You will see it stretch, twist and rip as it goes flat.

When mapping smaller areas — such as neighborhoods and cities — an assumption of flatness can be made.  It does depend on the risk of inaccuracy for the map’s purpose.  A few yards of inaccuracy may be fine for consumer level navigation.  A lower level of inaccuracy may be required for landing airplanes in the middle of a runway.  Property boundary disputes may need accuracy to within a few inches.

Some historically powerful island nations may prefer a Mercator projection to give the appearance of a larger land mass.  Just as with statistics, a projection’s distortion can be used to enhance or diminish a feature.

The Mercator projection is commonly used in maritime navigation because it accurately represents initial the shortest distance initial direct bearing.  The other “feature” of a Mercator projection is the size and shape distortion of objects increasing in distorted scale from the equator to the poles, where land masses appear significantly larger.

Why are they called projections?  Imagine a bare light bulb representing the Earth.  Take a piece of paper and hold it to the light.  The features of the Earth are projected on to the paper.  Holding the paper at different angles or shapes make different projections.  Draw a square, circle and triangle on the light bulb.  Now you’ll really be able to observe the distortion of these shapes.

The Mercator projection is a cylinder shape in parallel with the Earth’s axis.  Maps can be referred to by the projection, or by the property that is accurately represented.  Projection shapes include conic or plane (flat).  Preserved properties include direction (azimuthal), shape locally, area, distance, and shortest route.  There are hybrid maps that blend properties of different maps.  While no properties are kept accurate, it “looks” better and more accurate.  Again, it all goes back to the purpose of the map.

Overlay of three projections to show distortions.
Overlay of two projections to show distortions.

There are a few terms used in mapping and GIS that you need to know.

Latitude (aka parallels): These horizontal lines on a map parallel to the equator.  These can be remembered because you give people “latitude” to get their job done so they can go as far to the sides as possible but that doesn’t include a promotion or demotion.  Latitudes are names 0° at the equator to 90° North or South at the poles.  1° difference in Latitude is just about 69 miles.  Except for the equator, a circle of latitude is not the shortest distance between two points.

Longitudes (aka meridians): These vertical lines on a map go North South.  These can be remembered because of the Prime Meridian.  The Prime Meridian is 0° going through Greenwich, England.  The meridians run from there East or West to the International Date Line at 180°.  One the equator, 1° of longitude is ~69 miles converging to 0 miles at the poles.

Great Circle: The shortest distance between two points occurs along a great circle.  A great circle cuts a sphere into two equal halves.  A great circle is the largest circle that can be drawn on a sphere.  All meridians are great circles.  The only parallel that is a great circle is the equator.

Nautical Mile: One minute of latitude along any meridian is a nautical mile.  One nautical mile = 1.85 kilometers = 1.15 miles = 6076 feet.  Note that meridians are used because the distance along parallels change.  The distance along a meridian is the distance between parallels.


GIS continues : How far would you walk for a degree change?