# Sample

From OpenFOAMWiki

Sampling values from a simulation and writing them to a file is done by editing the `sampleDict` file into the `system` folder of the case you run.

An example of such a file is provided at the end of this article.

One must specify:

- An
`interpolationScheme`choosed in:`cell`for a sampling-value equal to the one in the cell-center (which is considered constant over the whole cell)`cellPoint`for a sampling-value interpolated according to the cell-center's value and to the vertices values (which are determinated by neighbouring-cell's centers values)`cellPointFace`is the same as the one before with the interpolation including also the faces' values (which are determined by the current face interpolation scheme for the field (linear, gamma, ...)

- A
`writeFormat`choosed in:`xmgr`for`jplot`for`gnuplot`for the OpenSource program Gnuplot (which is almost a raw format)`raw`for a raw format file

- A description of how to sample in the
`sampleSets`subset, i.e. which points of the system being simulated must be sampled. One can describe thoose points directly by using the`cloud`method or, maybe smarter, by selecting one of the methods further described.

Please note that the methods described belows all share 2 attributes that must be provided with the ones needed for the desired method:`name`which will be used to refer to the sampling with the current method and also for the filename produced`axis`which will be used do determine how to write the point's coordinate. Ways of doing it are the following:`x`(or`y`or`z`) if you only want the x-coordinate to be reported`xyz`if you want all coordinates to be reported`distance`for the distance to point`(0,0,0)`to be reportedThe methods are:

`uniform`: used to sample`nPoints`evenly distributed points on a line, providing its`start`and`end`points.`face`: used to sample points located at the intersections between a line provided by its`start`and`end`points and the faces (i.e. faces enclosing cells) of the system being simulated.`midPoint`: same as`face`but the sampling point is now the midpoint between 2 faces intersections.`midPointAndFace`:`face`and`midPoint`together`curve`: the coordinate list`points`is considered as a curve (i.e. an assembly of segments), the sampling points are the intersections between this curve and the faces (i.e. faces enclosing cells) of the system being simulated.`cloud`: the points of the coordinate list`points`are the one that will be sampled.

A description of which field is to be sampled. A field is always a scalar field, meaning that if you must sample a vector/tensor field you must sample all the coordinates separatly (see the sampleDict example at the article's end).