Sometimes a picture is worth a thousand words, and that is especially the case if you have ever read my verbose Geospatial Frequently Asked Question (G-FAQ) series, haha! So this month, I break from tradition with a video tutorial as the focus of this G-FAQ on color balancing rasters (or imagery) in ArcGIS 10.x. This is a topic that could take hours to cover, so this month I focus on one specific issue I have noticed with ArcGIS 10: the automatic color balance it applies to rasters.
Specifically, this G-FAQ will address this set of questions:
How do I know if ArcGIS applied a stretch to my imagery? How do I turn this stretch off? How can I make the stretch ArcGIS automatically applies look better?
Before you jump into the video posted with this G-FAQ, let me spend a bit of time introducing this topic. When you snap a picture on your cell phone, there is an involved process on the backend which takes the raw image and makes the colors look as ideal and realistic as possible. And as many of you have found out, this automated process is not always ideal – hence cell phone makers have given us the power to edit the images, for instance with the Auto-Enhance option on the iPhone. The process I just described is often referred to as: color balancing, histogram stretching or color enhancement (in the world of remote sensing and/or GIS that is).
Color imagery, including a cell phone photo, is created by combining the blue, green and red reflectance values from a surface. When speaking of satellite and aerial imagery specifically, these blue, green and red surface reflectance values can be reported in either 8 or 16-bit depth. While we do not have the time to cover this topic in full (see our January 2013 G-FAQ for a more detailed handling of bit depth), bit depth refers to the number of blue, green and red shades that are possible in an image. In 8-bit depth, there are 256 shades possible and then with 16-bit there are 65,536 shades. However, a computer monitor cannot read 16-bit depth images, so they must be converted to 8-bit depth to be visible. Before a 16-bit image is converted to 8-bit depth, it will look like an all black tile. Once converted to an 8-bit image (or even if you started with an 8-bit image), you should see colors in a program like ArcGIS when you load a raster but they might not be ideal. And so that is what color balancing is for!
Okay, now that we have introduced the topic, it is time to move on to the tutorial. In this video, I will load both 8 and 16-bit depth satellite images into ArcGIS 10.1 to show our readers what happens and then offer a few color balancing tips.
Do you have an idea for a future G-FAQ? If so, let me know by email at firstname.lastname@example.org.
Find Out More About This Topic Here
- Esri – Raster Stretch Functions
- Fort Lewis College – Digital Image Analysis Techniques III – Image Enhancement
- University of California, Irvine – How to Stretch Image Pixel Values
Brock Adam McCarty