Merge pull request #557 from OceanParcels/double_prec_doc

add message about coordinates accuracy to nemo3D

Author: delandmeterp

parcels/examples/tutorial_nemo_3D.ipynb

@@ -25,6 +25,8 @@
     "*What about grid indexing?* Since the C-grid variables are not located on the same nodes, there is not one obvious way to define the indexing, i.e. where is `u[k,j,i]` compared to `v[k,j,i]` and `w[k,j,i]`. In Parcels, we use the same notation as in NEMO: see [horizontal indexing](https://www.nemo-ocean.eu/doc/img360.png) and [vertical indexing](https://www.nemo-ocean.eu/doc/img362.png).\n",
     "It is important that you check if your data is following the same notation. Otherwise, you should re-index your data properly (this can be done within Parcels, there is no need to regenerate new netcdf files).\n",
     "\n",
+    "*What about the accuracy?* By default in Parcels, particle coordinates (i.e. longitude, latitude and depth) are stored using single-precision `np.float32` numbers. The advantage of this is that it saves some memory ressources for the computation. In some applications, especially where particles travel very close to the coast, the single precision accuracy can lead to uncontrolled particle beaching, due to numerical rounding errors. In such case, you may want to double the coordinate precision to `np.float64`. This can be done by adding the parameter `lonlatdepth_dtype=np.float64` to the ParticleSet constructor. Note that for C-grid fieldsets such as in NEMO, the coordinates precision is set by default to `np.float64`.\n",
+    "\n",
     "### How to create a 3D NEMO `dimensions` dictionary?\n",
     "\n",
     "In the following, we will show how to create the `dimensions` dictionary for 3D NEMO simulations. What you require is a 'mesh_mask' file, which in our case is called `coordinates.nc` but in some other versions of NEMO has a different name. In any case, it will have to contain the variables `glamf`, `gphif` and `depthw`, which are the longitude, latitude and depth of the mesh nodes, respectively. Note that `depthw` is not part of the mesh_mask file, but is in the same file as the w data (`wfiles[0]`).\n",