Hi Carl,
There is not a straightforward answer to setting breakpoints for color enhancements. What is chosen depends on the satellite channel and the important features in the image. The
COMET courses covering GOES channels at MetEd may be interesting to you. There are a few that cover the response of satellite channels to various features and also RGB tutorials. They are free and very useful.
First, the color tables RBTOP and JSL were posted in a format compatible with an old (still in use) software package for satellite imagery (McIDAS-X). In this software package, brightness values were scaled to temperature, and a bi-linear enhancement was usually applied.
Reading the documentation, the OSPO color tables you posted (JSL and RBTOP) are all using this bi-linear enhancement. In my code, the equation is written from temperature to brightness value so that I can pick out the color level I want and reorganize the color table according to temperature scaling. The OSPO website lists the conversion from brightness value to temperature. However, it is the same formula (see also:
http://www.goes.noaa.gov/ECWV1.html).
Therefore, the enhancement is a linear relationship from Brightness Value to Temperature for Brightness Values greater than 176:
If B > 176, T = 418 - B
The enhancement has a bi-linear relationship from Brightness Value to Temperature for Brightness Values less than or equal to 176:
If B <= 176, T = 330 - (B/2)
On the other hand, the Fog Depth Enhancement that is on
Gary Ellrod's GOES Enhancement Table page was using a stretch table, which apply a linear scaling of a temperature range (-13 to >6) to a brightness value (0 to 255).
To achieve the same color table without code, you could use the interactive color table editor in McIDAS-V (find:
Color Table Editor documentation in the
User’s GuideIn addition, if you have not yet worked through the Satellite Imagery Tutorial, you could work through an
example with the Problem Set #1 Solution to gain a better understanding of the process.
I will provide an outline of how I approached the problem:
1.) set the number of colors of the color table to 257. Do this by clicking the + arrows above the color bar in the color bar editor.
2.) Click
File->Save As-> to create a new color table name.
3.) Set the range to -13, 7
4.) Set breakpoints at the breakpoints listed in the table. In the case of the fog enhancement it is -13, -3, 2, 3, 4, 5, 6, 7. (To set breakpoints, right click above color table in color table editor and select
Add BreakpointAt Data Point5.) Click on each breakpoint and set the color of those breakpoints from the table. Note: Gary lists these breakpoints as Blue, Green, Red, but are Red, Green, Blue from top to bottom in the color table editor. Work in one direction. Once the breakpoint color is set, use the right-click
Edit Colors to fill either right or left to next breakpoint
6.) Continue to next breakpoint and follow the same procedure as was done at the previous breakpoint.
7.) When working from -13, 3, select interpolate rather than fill.
Give it a try. If it does not work for you, post again and let use know where you are struggling. You should be able to replicate a linear color table fairly well with the color table editor.
Here is a photo of what you completed color table would look like when it is done
This reproduces Gary's original example fairly well
I believe once you learn this procedure, you could reproduce most if not all the enhancements on Gary's page.
Hope this helps!
Joleen