# What does sprectrum1d remove trend (-L) do?

**URL:** <https://forum.generic-mapping-tools.org/t/what-does-sprectrum1d-remove-trend-l-do/2830>\
**Category:** Q&A\
**Created:** [April 18, 2022, 5:45pm UTC](https://forum.generic-mapping-tools.org/t/what-does-sprectrum1d-remove-trend-l-do/2830 "2022-04-18T17:45:08Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Esteban82](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/esteban82/32/54_2.png) [@Esteban82](https://forum.generic-mapping-tools.org/u/Esteban82)\
**Post date:** [April 18, 2022, 5:45pm UTC](https://forum.generic-mapping-tools.org/t/what-does-sprectrum1d-remove-trend-l-do/2830/1 "2022-04-18T17:45:08Z")

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In [spectrum1d -L](https://docs.generic-mapping-tools.org/dev/spectrum1d.html#l) says

> By default, a linear trend will be removed prior to the transform.

Which is the formula used to do that? This (taken from [trend1d first example](https://docs.generic-mapping-tools.org/dev/trend1d.html#examples))?

> To remove a linear trend from data.xy by ordinary least squares, use:  
> gmt trend1d data.xy -Fxr -Np1 \> detrended\_data.xy

Thanks in advance!

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**Author:** ![Esteban82](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/esteban82/32/54_2.png) [@Esteban82](https://forum.generic-mapping-tools.org/u/Esteban82)\
**Post date:** [April 19, 2022, 9:15pm UTC](https://forum.generic-mapping-tools.org/t/what-does-sprectrum1d-remove-trend-l-do/2830/2 "2022-04-19T21:15:37Z")

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pinging @pwessel.

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**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\
**Post date:** [April 19, 2022, 9:40pm UTC](https://forum.generic-mapping-tools.org/t/what-does-sprectrum1d-remove-trend-l-do/2830/3 "2022-04-19T21:40:16Z")

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Even without checking the code I bet it removes a least squares linear fit. What else.

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**Author:** ![Esteban82](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/esteban82/32/54_2.png) [@Esteban82](https://forum.generic-mapping-tools.org/u/Esteban82)\
**Post date:** [April 19, 2022, 10:16pm UTC](https://forum.generic-mapping-tools.org/t/what-does-sprectrum1d-remove-trend-l-do/2830/4 "2022-04-19T22:16:42Z")

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Yes, I agree. I just want to understand well how the module works.

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**Author:** ![pwessel](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/pwessel/32/30_2.png) [@pwessel](https://forum.generic-mapping-tools.org/u/pwessel)\
**Post date:** [April 20, 2022, 5:54am UTC](https://forum.generic-mapping-tools.org/t/what-does-sprectrum1d-remove-trend-l-do/2830/5 "2022-04-20T05:54:59Z")

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Least-squares fit of linear ax + b trend.