FAQ - General
What is geodesy?
Geodesy is a wide field of study that finds out information about earth, to assist all earth monitoring and mapping activities. It is such a broad topic as the various subjects such as time, gravity, mass, oceans, astronomy and measurement methods all affect each other.
[1] "Geodesy is the science that determines the figure of the earth and the interrelation of selected points on its surface by either direct or indirect techniques".
[2] Geodesy is a field of study involving measurement and mapping of the earth; study of the gravity field; "determination of the shape and size of the earth and it's orientation in space".
[1] Smith, J. (1997) Introduction to geodesy the history and concepts of modern geodesy, Wiley series in surveying and boundary control. Roy Minnick, Series Editor. Canada.
[2] Torge, W. (2001) Geodesy 3rd Edition, Walter de Gruyter GmbHand Co. Berlin, Germany.
Why don’t we just use WGS84? Why do we need local systems?
We are moving towards an international datum but it will be a long time before technology and resources make it possible. At the present time the world operates with a mix of old and new information on world mapping. Regions around the world have created their own local datums which provide accurate positional information for their area. These local datums are not necessarily compatible with other datums or over other areas. The datums have been created over millennia using the best technology and information available at the time so they vary in methodology and accuracy. However, they still serve as the basis of maps and charts around the world and they still provide a more accurate picture of their locality than WGS84. Australia’s datum GDA94 is very close to WGS84 and was designed to be compatible for most purposes. High accuracy measurements of centimetre precision cannot be determined with WGS84 but can be achieved with GDA94.
What happens to coordinates of points on the ground when the ground is moved by earthquakes or techtonic drift? Does Google Earth update for earthquakes?
They change! Coordinates such as latitude & longitude represent a measure of angles. It is not like naming a place. So when there are earthquakes we update the records of coordinates for known locations such as permanent marks, and recalculate their position as a measure of angles in relation to a datum (starting point & equations) which is usually the center of the earth.
Sometimes a 'deformation patch' is used over an area to try & update coordinates faster. See more information in Crook & Donnelly 2013 Updating the NZGD2000 deformation model & on the LINZ website under deformation models http://www.linz.govt.nz/data/geodetic-system/datums-projections-and-heights/geodetic-datums/new-zealand-geodetic-datum-2000.
While Google earth does update it's information, the effects of earthquakes will be not so noticable as the the accuracy is usually around 5 meters (be aware this is a general statement, not an accurate discripton of accuracy) due to the challenges involved in matching imagery with mapping data accross a global scale.
Why arn't coordinates fixed to points on the ground?
If the coordinates of positions on the ground were always the same the straight lines which make up our grids of latitude & longitude or Easting & Northing would become skewed. Coordinates are time dependant and are updated with new datums or in some cased the use of deformation models. General public do not usually notice these changes as they rarely use coordinates and if they are shown on maps they are often rounded which hides the meters of movement. The difference is also hidden on large scale maps as often meters are represented by millimetres on paper
See the model of warping grid lines
See the experiment of historical map comparisons
Why does the earth look like a perfect ball/sphere from space but we are told it is not?
" It is hard to show the separation between an ellipsoid and geoid at scale, as the undulations are so small relative to the size of the earth. If we imagine condensation from breath on a half meter metal ball - the earth is the metal ball, the oceans (which represent the geoid) and their depth are equal to the moisture from our breath on the metal surface." - Dr Tim Foresman
Consider taking a photo of the ocean from a hill far away. The ocean looks smooth, but if you were to take a boat and go out into the wide sea, you would find that it is not smooth at all, it is lumpy with waves & surges. Photos from space are a bit like this. They are taken from far away and the undulations of the ocean and ruggedness of the land is hidden by the sheer scale of earth's size.
The earth is wider around the equator and squashed at the poles, but the squashing is so small relative to the size of the earth that it is hard to see. The diameter of the earth is 40,075km at the equator (Wikipedia) whereas the squash is about 21km at the top and bottom. Refer to Wikipedia's article on the ellipsoid called GRS80 - https://en.wikipedia.org/wiki/GRS_80
What are datums? Why do we need them?
A datum is the thing we are referring to when describing a measurement.
It is the start point and the mathematical method used.
Consider this:
Measure a mountain from its base on the east side in meters, and
measure the same mountain from the base at the west side in feet.
Both measurements are describing the same thing - a distance from base to top. But one begins from one side of the mountain which is higher above sea level than the other and one shows 8,848m and the other shows 2,900 feet!
Think of a datum like a starting point for a measurement. If you were to measure your height you would start from the floor with a ruler and measure up from 0. If you are to give a description of a location on earth how would you do it? Some cleaver astronomers and scientists worked out a way to describe locations based on their location relative to the centre of the earth - or at least where they thought the centre was - this is a hard thing to work out.
Given the complexities of our earths shape & moving systems some people found it better to consider the centre point in a slightly different place to others. This gave them more accurate measurements in their town.
Back before satellites and our latest gadgets, countries in the world were somewhat separate and had their own ways of describing coordinates for a point. The exception being sailors, who have their own astronomical ways of getting around. This situation was fine at the time, we didn't have computer systems producing and mixing up large amounts of data with coordinates attached to it.
Since then, things have changed and people have to work more internationally and we have had to be considerate that maps might have different coordinates due to the methods used to make them. International efforts are creating worldwide mapping systems but there are still times when local measurement systems are needed.