The question about using grdfft to calculate geoid

Q&A
1

code:
gmt grdfft temp.grd -Ig -Ggeoid.grd

question
The calculated geoid magnitude is incorrect and differs significantly from the actual result. The document says that no additional scaling is needed, and I’m not sure what’s wrong with that.

(right panel is EGM2008 geoid model)

image
image1920×745 84.1 KB

2

What is temp.grd?

1 Like
3

A gravity anomaly model (unit: mGal)

here is the data
temp.grd.zip (549.2 KB)

4

This is the distribution of the gravity anomaly model. It’s right.

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image1920×1176 127 KB

5

Strangely, the deflection of vertical based on geoid is correct. geoid’s magnitude is not correct but its difference (deflection of vertical) is correct, I guess it could be removing the trend term, but using N+l is still not correct.

gmt grdcut grav_china_sea.grd -R130/140/15/20 -Gtemp.grd
gmt grdfft temp.grd -D -S1e4 -Gvgg.grd
gmt grdfft temp.grd -Ig -N+l -Ggeoid.grd
gmt grdfft temp.grd -Ig -S1e6 -A-90 -Gdovx.grd
gmt grdfft temp.grd -Ig -S1e6 -A0 -Gdovy.grd

6

I think @Joaquim says “if the script isn’t the problem, maybe it’s the data, how can we help you if we don’t have it?”

7

ok, I have uploaded the data.

8

I might be off-track, but doing -Ig converts the geoid to height (meters). Is that what you want or do you want to stay in mGal ?

9

Thanks, I used -Ig to calculate geoid from gravity anomaly, and geoid is what I want.

10

Hi,
I don’t think you can do this comparison. The EGM is a global grid and you cannot expect to capture all the wavelengths when computing the the geoid from a small region as your faa grid. And extending that grids area is limited to the extent that a FFT is valid when applied to a geographical grid. I also don’t think there is an error in grdfft. As the main page says:

This is equivalent to divide by kr in the frequency domain  (kr is radial wave number).

and that has been tested during years (ofc nothing is bug-free but …)

11

:grinning_face: OK, I got it. Thanks for your reply !