Datums and projections

This lesson is about projections and datums.

These two properties determine how we model the planet so that we can represent it.

So, what does the Earth look like? What shape is it?

There are many different ways to represent the Earth. They all have uses in different contexts.

We often think of the Earth as a sphere.

Ita sempre hanoin mundu nee kabuar ida.

Actually, the earth is more closely shaped like a slightly squashed orange. The mathematical model of this is called a spheroid.

Tuir lolos, mundu nee nia modelu besik hanesan sabraka kabuar, tuir matematiku ba modelu nee hanaran spheriod (kabuar)

This is a very easy mathematical way to represent and model the earth.

Ida nee sai maneira matematiku fasil liu hodi reprezenta no modelu ba mundu.

If you map the earth using the relative strength of gravity, it looks more like a potato!

Karik ita halo mapa ba mundu forsa relativu ba gravidade, nia sai hanesan tiha fehuk-ropa ida.

Mathematically, this is much harder to represent.

Matematikamente, ida nee sai araska-liu atu reprezenta.

Much effort has been made to create a mathematical model of the Earth. This is called the geoid.

The geoid is not smooth. It has peaks and troughs. It does a good job of representing the Earth in a mathematical model.

It is used as the reference for WGS 84. WGS 84 is the reference used by Global Positioning System(GPS) to determine location on the planet’s surface.

So, the earth could be said to have three different surfaces:

Nune’e, mundu iha superfisie (permukaan) tolu maka diferente:

1.the ground level or terrain, 
nivel fundu du terra ou terrain

2.The ellipsoid or the simple mathematical surface, and 
Kabuar naruk ou simples matematiku superfisie no

3.The geoid which is the complex mathematical surface. 
Geoid mak sai superfie matematku kompleksu.

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It is important to understand the difference between the three surfaces. It is also important to note that they can each be either higher or lower than the other ones, depending on local conditions.

Datums

Dadus

Over the years, the countries who were developing modern mapping each created a mathematical model for there part of the world. Each of these differed slightly as they were designed to account only for the country who was producing the model. These models became datums.

Tinan hirak liu ba, nasaun hirak nee’ebe dezenvolve mapa moderna ida-idak kria modelu matematiku ba parte hosi mundu nee. Ida-idak hosi mapa hirak-nee dezeiñadu hodi konta deit-ba iha nasaun ida nee’ebe mak produs modelu nee. Modelu hirak nee bele sai dadus.

Unfortunately, they didn’t quite fit when you went to a different location.

Infelizmente, mapa hirak nee la-dun los wainhira ita to’o iha fatin diferentes.

This diagram shows the difference between datums.

Diagrama ida nee hatudu deferensia entre dadus.

If you were in Texas and used the Tokyo model for the shape of the Earth, you could think you were 800 metres away from where you were actually standing!

Karik ita iha Texas no uza modelu Tokyo konaba Mundu nia forma, ita bele hanoin katak iha iha 800 metrus liuba hosi fatin ne’ebe ita tour lolos hamrik hela dadauk.

Sumber: DNTPSC GIS LESSONS