# Calculate scale of a raster

A raster has a cell size of 200 meters. What is the corresponding scale of the raster?

New to GIS, using ESRI, semester is almost over but this one question is confusing. I didn't think you could calculate the scale of the raster in that you are given cell size of 200 meters and to my knowledge, or lack there of, the corresponding scale would be whatever the data set is using or that you adjust the scale to.

• scale is a ratio x to y, so the question is 200m is to what? is it 20cm is to 200m? that's 0.2m to 200m which is 1:1000. – Michael Stimson May 5 '15 at 3:27
• Michael, this was my original question back to my professor because I felt there weren't enough details. Even the actual questions is vague like "what kind of scale?" I ended up assuming it was map scale..Thanks so much for your reply!! – Keith May 5 '15 at 4:28

Rasters don't really have a scale, they have a resolution, which in your case is 200m. You can calculate an appropriate viewing scale for a given raster resolution based on the ability to distinguish features.

The ArcGIS Blog post On map scale and raster resolution discusses this:

In 1987, Waldo Tobler, renowned analytical cartographer (now emeritus from University of California-Santa Barbara) wrote, “The rule is: divide the denominator of the map scale by 1,000 to get the detectable size in meters. The resolution is one half of this amount.”

(snip)

So here is the appropriate resolution for a select set of map scales:

For example, if you were not sure what resolution imagery you needed to acquire to detect features at a map scale of 1:50,000, using Tobler’s rule above, you can determine that imagery of approximately 25m [50000/ (1000*2)] resolution would be sufficient.

Similarly, if you need to find out the mapping scale from a known imagery resolution you can do so using the formula below:

Map Scale = Raster resolution (in meters) * 2 * 1000

Two, as used in the equation above, doubles the resolution. Then that number is multiplied by 1000 to get the map scale.

Here’s an example. Say you have a raster with a resolution of 30 meters. Each pixel is 30 meters on a size (an area of 900 square meters). You double that to get four pixels (two rows and two columns) with a resolution of 60 meters on a size (an area of 3600 square meters). Then you multiply that 60 meter resolution by 1000 to get a map scale of 60,000.

(snip)

• Oh my gosh how in the world does this life function!! The exact reference man this is crazy! Thanks to you also Luke – Keith May 5 '15 at 4:24
• That's news to me.. and I've been doing GIS since '92! but it actually makes sense. It's not the 'scale' of the raster but the scale at which the raster can be reliably used. – Michael Stimson May 5 '15 at 4:31
• "the scale at which the raster can be reliably used..." for the given raster cell size correct? (in my case 200 meters) – Keith May 5 '15 at 4:38
• Actually this is my first class in GIS at UNLV. I've only just learned about photogrammetry...last week I learned how to digitize polygons – Keith May 5 '15 at 4:45
• No, sorry @Luke, I don't see stereo (just gives me a headache). I started with Telstra capturing assets on Unix using ArcInfo. (got paid Data Entry, level 1 : same as cheque processors - ArcInfo is only one program.. right?) – Michael Stimson May 5 '15 at 4:45

according to our good friend "Tobler," whom stated in order to calculate map scale, multiply the raster resolution (which should be in meters) by 2 multiplied by 1000. So in my above question my equation ended up like this here:

map scale = 200 meters x 2 x 1000

map scale = 1:400,000

there ya go

of course all great data sources deserve the appropriate credits so after juggling the reverse of another equation I sort of derived the answer I need! Thanks ESRI, in class and for lab instruction. Thanks Rajnagi!!

the reference I used was originally posted by rajnagi on December 12, 2010

"On map scale and raster resolution"

By Rajinder Nagi, Esri Cartographic Product Engineer

http://blogs.esri.com/esri/arcgis/2010/12/12/on-map-scale-and-raster-resolution/

• Ha! Beat you by a minute ;) – user2856 May 5 '15 at 4:26
• Wow, this is freaking crazy because I actually said to myself "dang, Luke beat me by a minute!" Thanks again for sure,because this solidifies my reference as well as my answer! – Keith May 5 '15 at 4:29