I wonder the meaning of reference pixel

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oaeen
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I wonder the meaning of reference pixel

by oaeen » Mon May 23, 2022 8:48 am America/New_York

The data I'm processing is from MODIS. The product I'm using is SeaDAS.

I want to reproject the L2-Level data from MODIS. When I was setting the output parameters, I'm confused of the meaning of reference pixel.

What's the effect of the reference pixel?
Where should I place the reference pixel?
What does the meaning of 'Easting' and 'Northing'?

My area of interest is:
Lon: [-75 -69.5]
Lat: [39 42]
( i.e. U.S.LongIsland )
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rapsodia86
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Re: I wonder the meaning of reference pixel

by rapsodia86 » Mon May 23, 2022 10:41 am America/New_York

Here is a software description and the guideline:
https://seadas.gsfc.nasa.gov/help-7.5.2/visat/Reprojection.html#crsParameters
"GUI: Output Parameters" section has a detailed explanation.

OB SeaDAS - xuanyang02
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Re: I wonder the meaning of reference pixel

by OB SeaDAS - xuanyang02 » Mon May 23, 2022 11:05 am America/New_York

Here is the help page on Reprojection Output Parameters -- https://seadas.gsfc.nasa.gov/help-8.2.0/desktop/ReprojectionOutputParametersDialog.html

oaeen
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Re: I wonder the meaning of reference pixel

by oaeen » Mon May 23, 2022 11:18 am America/New_York

Thank you for your reply.

I still don't know the real meaning of the northing and easting. Is there any explanation?

rapsodia86
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Re: I wonder the meaning of reference pixel

by rapsodia86 » Mon May 23, 2022 11:52 am America/New_York

From:
https://seadas.gsfc.nasa.gov/help-7.5.2/visat/Reprojection.html#crsParameters under "GUI: Output Parameters":

"Easting
This is the distance away from the origin (x component) to place the reference pixel. This essentially has the effect of shifting the geographic boundaries of your output file sideways.

Northing
This is the distance away from the origin (y component) to place the reference pixel. This essentially has the effect of shifting the geographic boundaries of your output file up and down.

A bit more about the Easting and Northing fields and the Reset button
The Easting and Northing fields can be a bit intimidating and not so intuitive, however basically you need to ask yourself whether you want the geographic boundaries of your output file to be adapted to "best-fit" and include all the data from the input scene, or whether you have a specific and exact geographic boundary for your output file in mind.

For the former "best fit" case, SeaDAS can calculate default Easting and Northing values based on your scene boundaries and CRS criteria. To do this, click the "Reset" button at the bottom of this GUI window. Of note is that the very first time you open this sub window during a SeaDAS session, the default will be calculated and applied, but from that point onward the assumption is that the value in the textfield is the value which you want to use If you make further changes to the CRS criteria or change to a different file, then you will again need to click "Reset".

For the latter case, since the false easting and false northing fields also appear with the CRS definition it may be better or perhaps more clear to set them there and then set the values here to 0.0 for each. Either way, keep in mind that the actual false easting and false northing is the sum of the corresponding values in the CRS parameters and Output Parameters.

Note: Easting, northing and the pixel size are given in the units of the underlying map (e.g. dec. degree for geographic and meter for the UTM projection)."

oaeen
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Re: I wonder the meaning of reference pixel

by oaeen » Mon May 23, 2022 10:00 pm America/New_York

Thank you for your reply!

Let's say the reference pixel.

Where should I place the reference pixel?
What's the difference between placing the reference pixel at scene center and at scene upper left?

OB SeaDAS - knowles
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Re: I wonder the meaning of reference pixel

by OB SeaDAS - knowles » Tue May 24, 2022 2:11 pm America/New_York

The default settings of the Reproject Tool are fine if you wish to reproject the source scene while retaining it's original boundaries and resolution. However, if you wish to have the Reproject Tool crop your scene into a specified region and/or adjust the resolution, then modifying the "Output Settings" can do this.

The reference pixel is the pixel index of the output (reprojected) image which aligns to your specified easting and northing values.

You can place the reference pixel wherever you want. But since you have a specific region in mind, using the upper left corner for reference is easier to explain (with less math)

You can modify the following to your particular region, but let's say I have a region (the Chesapeake Bay) and I want to use the geographic boundaries:
North 39.5
South 36.5
West -77.5
East -75.5

I would then set (with reference being upper left):
Reference pixelX = 0.5
Reference pixelY = 0.5
Easting = -77.5
Northing = 39.5

Note: Reference pixelX and Reference pixelY are 0.5 instead of 0 to account for the fact that the geo coding of the source is center of pixel and we want to include the remaining half of the source pixel in the output image.

Then for resolution, let's say I want 0.01 degrees per pixel (which is 100 pixels per degree). Since my region is 3 degrees in height and 2 degrees in width, then I would set:
Pixel sizeX = 0.01
Pixel sizeY = 0.01
Fit product size = false
Width = 200
Height = 300

Note:
Width = abs(West - East) * Pixel sizeX
Height = (North - South) * Pixel sizeY


Alternatively I could do reference at center in which case it would be:
Reference pixelX = 100.5
Reference pixelY = 150.5
Easting = -76.5
Northing = 38.0
Pixel sizeX = 0.01
Pixel sizeY = 0.01
Fit product size = false
Width = 200
Height = 300

I ran this on the level2 file:
https://oceandata.sci.gsfc.nasa.gov/ob/getfile/A2010283180500.L2_LAC_OC.nc

My reprojected chlor_a image (using the above parameters) is:
A2010283180500_L2_LAC_OC_chlor_a.png
A2010283180500_L2_LAC_OC_chlor_a.png (45.55 KiB) Not viewed yet

Here's the settings I used:
reproject2.png
reproject2.png (73.17 KiB) Not viewed yet

marilynplummer343
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Re: I wonder the meaning of reference pixel

by marilynplummer343 » Mon May 30, 2022 6:51 am America/New_York

rapsodia86 wrote:
> Here is a software description and the guideline:
>
> https://seadas.gsfc.nasa.gov/help-7.5.2/visat/Reprojection.html#crsParameters
> "GUI: Output Parameters" section has a detailed explanation.
I can also recommend the site https://create.vista.com/colors/color-names/%d1%81hambray/

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