The Wild World of **Geodesy!**

Go to http://lyzidiamond.com /geodesy to follow along.

What is *geodesy*?

Wikipedia says: **the scientific discipline that deals with the ***measurement* and *representation* of the Earth, including its gravitational field, in a three-dimensional time-varying space.

Let's **break it down**.

Yes. :)

But the Earth is *not* a sphere. It is its **own fancy shape**.

We *measure* and *describe* that shape in **a few ways.**

First: the *geoid.*

"Lumpy Earth"

The **geoid** is the shape of the Earth without *land masses, tides,* and *wind.* It assumes that all locations have the **same gravitational potential.**

Gauss described it as **"the mathematical figure of the Earth."**

But the *lack of uniformity* makes it hard to use as a **reference for measurement.**

Enter the *reference ellipsoid*.

or

"oblate ellipsoid."

"oblate ellipsoid."

The **important** part is that the *short (polar) axis* is roughly aligned with the *rotation axis* of the Earth.

Because we, as **cartographers** are engaged in *measuring* the Earth. We need to know **what goes where.**

So we now know that the *reference ellipsoid* for Earth is a **significant approximation** of the *geoid*, which is itself a **significant approximation** of the *real shape* of the Earth.

But the *reference ellipsoid* is **super helpful** to us when it comes to knowing *where things are* in **relation** to each other.

It enables the creation of a **geodetic datum**: a *coordinate system* and set of *reference points* that are used to locate **places on the Earth.**

A **datum** provides *starting points* for measurement.

Let's *walk through* an example.

You and I are standing just to the left of **this hill.**

We each take **fifteen** steps forward.

If you look at the *photo*, I am closer to the **right** side of the photo than you are.

We tend to think about *measuring space* in a **Cartesian** fashion.

Cartesian coordinate system

But *remember:* the Earth is **three-dimensional** and **lumpy,** even in the *reference ellipsoid.*

If you looked at our *relative positions* on and next to the hill from **above**, it will seem as though I *traveled further.*

But I *didn't.*

A **datum** takes this into account and makes a *coordinate system* that (roughly) reflects it.

Not perfect, but at least not flat.

Notice in that image where it says **Local Datum?**

There are **many different** datums that refer to **many different** reference ellipsoids.

For example, a datum may **define a location** for the *Equator*, but that location may be **different** for a *different datum.*

Because, again, of the **irregular shape** of the Earth.

Some *ellipsoids* are better for **certain areas**.

This is the **datum** that is used by the *Global Positioning System (GPS)*.

Going back to the **hill** example...

If I used my *GPS* to get to a **point** on the other side of the hill, it would give me **different** directions if I was going *over* the hill than if I was going *beside* the hill.

Using **WGS84**, it knows the relationship between *distance on the earth* and **relative location**.

But even if there *wasn't* a hill, this would still be **necessary.** Why?

Because the Earth is *three-dimensional.*

And *our job* as cartographers is to take this **three-dimensional** thing and represent it on a **two-dimensional** plane.

Because a **map** fits in our pockets *much more easily* than a **globe.**

And imagine how *big* that globe would have to be to be **useful** in a city!

Which brings in *another* important part of **geodesy:**

But first...

While datums allow us to **understand locations** in reference to each other on a *three-dimensional plane* ...

... **projections** are all about how we *show* those locations on **maps.**

Let's talk about **Gauss**, baby.

Gauss figured out that a *sphere* is one of the **only** shapes that cannot be *dismantled into a plane.*

Think about it like **peeling an orange.**

Omnomnom.

It is *impossible* to take that orange peel and **lay it flat** on the table.

So to display the *Earth* on a **page**, we need to *stretch* it.

Size.

Shape.

Position.

Direction.

Let's start with something *familiar*.

Hi, Mercator!

It became *popular* because of its strength in **nautical navigation**.

Every projection has **distortion**, but *Mercator* is a particularly **acute offender.**

Most notably, Mercator distorts *relative size* of land masses towards the poles.

Stretchy!

Another common projection is the **Gall-Peters projection**.

There are an **infinite** number of map projections.

Head to jasondavies.com /maps/transition

Which **projections** do you *like best?* Why?

MerCATor!

To create a **projection**, you need to *select an Earth shape* (like the **reference ellipsoid**) and then determine a *transformation* of points to a **Cartesian** plane.

Those **transformations** relate to a *projection surface*.

Transverse Mercator

Think about it like you're **wrapping** the globe with one of these *surfaces*, then **unrolling** it onto a *plane.*

In addition to the *projection surface*, you also have to decide on **aspect**.

In addition to classifying projections by **surface**, we can classify them by *distortion*.

I am going to **stop** here because my head is about to *explode* with geometry.

In the **next installment** of *geodesy at #maptime*, we will talk about Spherical Mercator and web mapping.

We'll talk about *great circles* and *rhumb lines* and the *common problems* in **geodesy**.

But for now:

We learned about **the science of geodesy!**

We learned about **geoids and reference ellipsoids!**

We learned about **datums and coordinate systems!**

We learned about **Gauss!**

We learned about **map projections and distortions!**

We learned that **this is a HARD PROBLEM.**.

Pat yourself on the back! *You are a champion!!!*

There is **so much** I didn't cover. A *quick Google search* will yield **so much more.**

Keep learning! *It just gets more fun!*

Thanks!

My name is Lyzi! I am on Twitter at @lyzidiamond! MaptimeOAK is on Twitter at @MaptimeOAK! Holler at us!