Hi Paul,

I’m interested in reproducing Gavin Simpson’s example on soap film smoothers and lake bathymetries (URL: https://www.fromthebottomoftheheap.net/2016/03/27/soap-film-smoothers/) using brms.

His model structure is as follows:

```
library(mgcv)
m2 <- gam(-depth ~ s(os_x, os_y, bs = "so", xt = list(bnd = bound)),
data = depth, method = "REML", knots = knots)
```

where bound is a list of spatial points (x,y) representing the lake perimeter to constrain the predictions and knots is a data frame of spatial points (x,y) representing a sequence of grid points that fall within that boundary.

What would be a comparable model structure using brmsformula syntax? I haven’t been able to figure it out on my own yet and am looking for your insight.

Thanks a ton,

Denise

- Operating System: Windows
- brms Version: 2.9.0