## Meaning of "X = NOTHING Y = EASTING"

Oil and natural gas exploration -- geology and geophysics
gfname
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Joined: Thu Dec 31, 2020 4:16 pm

### Meaning of "X = NOTHING Y = EASTING"

In a SEGY (contains field records of shot gathers) file's textual file header, it says "X = NOTHING Y = EASTING RIGHT-HAND Z-DOWN COORDINATES". What would "X = NOTHING Y = EASTING" mean? If it means a coordinate system of RIGHT-HAND Z-DOWN, why would it say both? Or X and Y directions here mean something else?

The file is downloaded from http://s3.amazonaws.com/open.source.geo ... ometry.sgy.

GuyM
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Joined: Sat Mar 24, 2012 11:35 pm

### Re: Meaning of "X = NOTHING Y = EASTING"

Hopefully it says NORTHING rather than NOTHING?

Normally you would have X as the EASTING coordinate and the Y as the NORTHING; maps usually have north pointed along the Y-axis.
That's a left-handed dataset - if you hold up your left hand and stick out your forefinger and thumb in an L shape, then your thumb is the Easting and the first finger the northing. Stick out your first finger at right-angles to those and you have the Z-axis.

This dataset - well it's a bit unusual; they have flipped the grid so that X is Northing and Y is Easting. Can't make that work with your left hand (lower left origin), you have to use your right hand (lower right origin). The Z-axis (second finger) can still point down however.

It happens; it's possible to mess up your geometry grid and have the whole thing flipped relative to the usual lower-left origin....

gfname
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Posts: 2
Joined: Thu Dec 31, 2020 4:16 pm

### Re: Meaning of "X = NOTHING Y = EASTING"

Yes, it says NORTHING rather thing NOTHING. Sorry for the typo.

Thanks for the explanations. It makes sense now.

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