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 🙂
It’s a very nice map you used in the example.
I wonder who made it? 😉
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.
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.
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!!!
That is awesome! Glad I could help!
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.
Good job! Maybe try Markly from http://marklyapp.com
Thank you! Simple and effective, unlike the long, drawn out explanation on Adobe’s site that I still don’t understand
Hi, The measuremeant scale is ticked in both your image and figure (you say to have it UNTICKED).
Measurement scale gives the mm, not the pixels
Looks like everyone else worked this out too.
Thanks heaps for this BTW
Thanks, I didn’t even notice!
Thank you so much! I’m doing a mural and trying to work out how many litres of each colour I need to mix. This has helped me to be economical with paint without guessing!
When I first found out how to do it, I was like “THIS IS MAGIC”.
Glad I could help 😀
genial!
thx a lot
Thank you for this! I used this in my thesis to measure distribution of road space in cities, it made the process a lot easier!
Thank you for the great tip! Can you suggest how to do this if you don’t have the extended version? Without the extended version I’m unable to set the measurement scale, so now I’m stuck with the rule set to the inches. Can I make an additional conversation based on the pixel per inch setting? Is that the same as the px? Thank you so much!
Yep, that should work!
Very lucid and succint. Rarely one comes across a solution to a seemingly intractable problem….. that works perfectly the first time. Like magic! Thanks a lot.
nice tutorial, but i will add something, may be this help for u guys. The number of pixels 157529 are not correct, this may give wrong calculation of area. To get correct number of pixels, click triangle at the top right corner, this will free up any engaged memory or previously existing pixel counts from photoshop memory, and will give REAL number of pixels. Remember, this triangle appears only when we have bigger selected area on the images. for small selections the triangle dont appear.
to get correct pixel count, click the triangle present on the top right corner of Histogram window.. now you will get correct number of pixels ..
my L1 measurement (1.793px) was smaller than the actual dimension (3ft) it corresponded to. so when i divided them, the ratio result was less than 1: 0.5018px per ft. this means that when i squared it to see how many px per square foot, i got an even smaller number: 0.2518 px.
what am i doing wrong? i’m sure it’s an easy solution, i just can’t see it. help?