# Mathematics for GIS Professionals

With a simple sum that only has two numbers and one single operation, or sign,

it’s easy to see how to calculate the answer. Either you add, subtract, multiply, or divide.

But what about when there are several numbers, and different operations?

Maybe you need to divide and multiply, or add and divide. What do you do then?

Fortunately, mathematics is a logic-based discipline. As so often,

there are some simple rules to follow that help you work out the order in which to do the sum.

**Rules of Ordering in Mathematics – BODMAS**

**
**BODMAS is a useful acronym that lets you know which order to solve mathematical problems

(or sums). It’s important that you follow the rules of BODMAS as without it your answers can

be wrong.

The **BODMAS** acronym is for:

**B**rackets (parts of a calculation inside brackets always come first).

**O**rders (numbers involving powers or square roots).

**D**ivision

**M**ultiplication

**A**ddition

**S**ubtraction

**Brackets**

Start with anything inside *brackets*, going from left to right.

Example:

$4 × (3 + 2)=$

You need to do the operation, or sum, inside the brackets first, $3 + 2$, then multiply

the answer by $4$.

$3 + 2 = 5$.

$4*5=$

If you ignored the brackets and did the sum $4* 3 + 2$ you would get $14$.

You can see how the brackets make a difference to the answer.

**Orders**

Do anything involving a power or a square root next (these are also known as *orders*),

again working from left to right if there is more than one.

Example:

$3^2 + 5$

You need to do the power sum first, before you can add $5$.

$3^2 + 5$

$9 + 5$

**Division and Multiplication **

Once you have done any parts of the sum involving brackets or powers the next

step is *division* and *multiplication*. These rank equally, so you go from left to right in the sum,

doing each operation in the order in which it appears.

Example:

$4*5 ÷ 2 + 7 = $

You need to do division and multiplication first, but you have one of each.

Go from left to right, which means that you start with $4*5 = 20$.

Then do the division, $\frac{20}{2} = 10$. Only then do you move to the addition:

$10 + 7 = 17$. The answer is $17$.

**Addition and Subtraction**

The final step is to calculate any *addition* or *subtraction*. Again, these rank equally,

and you simply move from left to right.

Example:

$4 + 6 – 7 + 3 = $

You simply start on the left and work your way across.

$4 + 6 = 10$

$10 – 7 = 3$

$3 + 3 = 6$

The answer is 6.