What are the raster functions applied to the living atlas chlorophyll-a data?

[DuncanHornby]

Esri,

I'm working on a project that requires me to access the chlorophyll-a living atlas dataset here. I am accessing it via the portal tab in the ArcPro catalog pane.

As I wish to do some basic mean values over specific months I am using the Analytic renderer which in ArcPro says it is a log10 scale, but this hides the original pixel value. 

You can see the raw pixel value if you change the pop-up configuration to show it.

But these values do not appear to be exposed to the aggregate multi-dimensional tool. SO when I run that tool I aggregate over the log10 values.

After asking an AI I have learnt that a raster processing template is applied to transform the values to a log10 scale and the renderer shows this as 0 to 100.

No doubt the mathematicians amongst you would say you can reverse engineer the transformation to get to the original chlorophyll-a concentrations but after querying AI it suggests that the common equation used has a gain/offset value which it took a best guess at. I show the response of the AI below but there is no information I can find in the metadata of this Authoritative dataset that describes the exact processing template and the functions used with their settings.

So my question is simply what are the raster processing functions applied to the chlorophyll-a data? In what order and what are the settings? I need to reverse engineering this log scale to see the real values for my colleagues analysis to continue.

The AI response was this:

2. The Conversion Formula

In many oceanographic and satellite products (like those from MODIS or Sentinel), the relationship between the Raw Pixel Value (PV) and the Physical Value (V) follows a specific linear-log transformation.

The most common formula used in these analytic renderers is:

V = 10^(a . PV + b)

Where:

• PV: The raw service pixel value (0.207422)

• a: A scaling slope (often 6.75 or 7.0 in standard ocean color products)

• b: An offset (often -0.9 or -1.0)

Let's test the math:

If we plug your values into a standard scaling used by many Chlorophyll-A services:

1. Scale the raw value: (6.75 * 0.207422) + 0.05 = 1.4501

1. Apply the Power of 10: 10^1.4501 = 28.19

Your result of 27.4969 is remarkably close to this. The slight difference comes down to the specific Gain and Offset parameters defined in the metadata of your specific raster service.

[HongXu · Accepted solution]

You do not need to reverse engineer. To use unprocessed pixels, simply choose the None template ( available from layer's Data tab->Processing Template), then use this as the input to geoprocessing tool.

[DuncanHornby]

@HongXu  thank you for the answer that is the solution to get to the raw pixels values!  I must say "A No-Op Function" statement is not very helpful whilst the Analytical and Cartographic renderers provide informative advice. Please could you feed that back to the ArcPro development team. A simple explanation of what that means would be very useful in that Processing Template dropdown. If I search esri help for "A No-Op Function" it returns nothing, so I have know way of finding out what it is...