Measuring Area on a Map with Photoshop

Here’s a quick guide I wrote on how to use Photoshop to easily measure an area on a Map using Photoshop.

In this guide I’m using Photoshop CS5 for Mac, most of the concepts will be similar for the other versions.
First up: Scan your map and get it into Photoshop

Working out the Scale

1. If you’ve got a scale marker on your map, use that, otherwise you’ll need to measure a distance that you know, in this case I used a basketball court (marked as 20mx33m).
2. Use the Ruler Tool in Photoshop (hidden under the eyedropper tool) and select the distance for your scale.

3. In the ruler tool options (top of the screen usually), make sure “Use Measurement Scale” is unticked. Then look at the L1 value. Write this number down!

4. In my example I got the value 246.88px. So from this we can work out that there’s 246.88 pixels in 20 meters.
This gives us 12.344 pixels per meter and if we square it we get 152.374px per square meter.

Highlighting the Area

1. Make a new layer, and using a wonderful bright colour, paint over the area you want to calculate the size of. Make sure you set the brush to have a hardness of 100%.

2. Use the Magic Wand Selection Tool to select the area you just painted. Set the tolerance to 1 and untick Antialias and Contiguous (it should select everything you’ve painted).
3. Go to the Window Menu and bring up the Histogram. If it doesn’t have the numbers underneath it, click the drop down box in the top right hand corner of it and go to Expanded View.

4. Change the “Source” dropdown box in the Histogram window to Selected Layer and write down the number under Pixels. I got 157,529px.

Doing the math

So we know that one square meter is 152.374px and our selected area is 157,529px.
So we do some quick division: 157529/152.374 and we get 1033.83meters^2

If you used this or have any corrections, please leave a comment 🙂

Posted by duck in Home, How-To Guides, 11 comments