Duck's Tech Blog

Measuring Area on a Map with Photoshop

by on Aug.15, 2012, under Home, How-To Guides

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 :)

:

7 Comments for this entry

  • Jan

    It’s a very nice map you used in the example.

  • duck

    I wonder who made it? ;)

  • Ken

    Very helpful, thanks! My landscaper wanted to know how much grass to get so I marked up a plot plan with where the grass is going, painted it green, and then followed your tips using the histogram – definitely something I would have never figured out on my own.

    For how to select all the content on the layer, rather than using the magic wand to pick up all the parts of the grass, I did a select all and then switched to the move tool and nudged one pixel left and one right – a wiggle. That reduces the selection to all the content of the layer.

  • Hhins

    Thanks for the great tip, just ran into one problem – my value was way off but I couldn’t figure out why… (may be due to a large canvas size) so I had to hit the little refresh on the histogram box and it updated.

  • Ben

    We look for dilated or collapsed bloodvessels in tissue microscope slides. We therefore need to know the volume of the lumen of each visible vesel. A count up of all these volumes of each slide gives an idea whether this part of the tissue has more or less collapsed (or dilated) vessels.
    Your tip helped us in determining these lumen volumes.
    Thanks a lot for that!!!

  • duck

    That is awesome! Glad I could help!

  • SJ

    Thank you very much for the awesome tip!

    This is really useful for me as a student researcher, with no access to a lot of site information that is often kept confidential by developers.

Leave a Reply

Looking for something?

Use the form below to search the site:

Still not finding what you're looking for? Drop a comment on a post or contact us so we can take care of it!

Visit our friends!

A few highly recommended friends...