# [Proj] Understanding deformation model usage

Kristian Evers kreve at sdfe.dk
Tue Feb 27 04:35:49 EST 2018

```Steve,

I was considering just calling them t_a and t_b or something like that to illustrate that what really matters is the difference between the two since it is all relative. Which t comes first dictates how the grid is applied (added or subtracted).
The way I think about it is that we are moving a coordinate at time t_obs backwards or forwards in time to t_epoch. In the example in the docs the last step is applying the deformation model on a coordinate at t_obs=2000.0 which we want to move back in time to t_epoch=1994.704 (which is the epoch of the Danish ETRS89 realisation). After the deformation model has been applied we have subtracted ~5 years' worth of land uplift.

I hope that helps you understand this better.

/Kristian

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Sendt: 27. februar 2018 09:34
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Emne: Re: [Proj] Understanding deformation model usage

I can see t_obs is always the *grid_epoch, except when you do an inverse (by
swapping t_epoch and t_obs).

stevejking wrote
> Kristian,
> I think I follow your (in development) user guide example here. Very
> useful
> thanks.
> https://kbevers.github.io/usage/operations/transformations/deformation.html
>
> But if the velocity shifts are tied to Cartesian coordinates, surely this
> is
> only the average velocities at the coordinate at a moment in time. There
> is
> no parameter to specify a *grids_epoch.
>
> Eg. What to do If my grid is Cartesian coordinates and velocities at epoch
> 2010.0, t_obs=2000.0 and t_epoch=2017.0?
> Can be achieved by ensuring in individual steps that t_obs or
> t_epoch always equals *grids_epoch ? I think that’s what your example
> does?
>
> *grids_epoch is a made up option to illustrate.
>
> Steve
>
>
>
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