# HydroGeoSphere/Selecting Elements

We can use the following instructions to alter the set of chosen elements.

## Clear chosen elements

All elements in the domain will be flagged as not chosen. This is recommended if you are unsure of which elements are chosen due to previously issued instructions.

## Choose elements all

All elements in the domain will be chosen. This is useful if you wish to assign a property to all elements in the grid.

## Choose elements by zone

* nval Zone number.
Elements which have zone numbers equal to the given value are chosen.

## Choose elements by zone, within overlay

* nval Zone number.
* fname Name of the GRID BUILDER overlay file.
Elements which have zone numbers equal to the given value and whose centroid lies within the first group of entries in the overlay file are chosen.

## Choose elements x plane

* x1 The x-coordinate of the plane.
* ptol Distance from the plane.
Elements within distance ptol of the plane defined by the equation x = x1 will be chosen.

## Choose elements y plane

As above but for the y-plane.

## Choose elements z plane

As above but for the z-plane.

## Choose elements 3pt plane

* x1, y1, z1 xyz-coordinate of the first point.
* x2, y2, z2 xyz-coordinate of the second point.
* x3, y3, z3 xyz-coordinate of the third point.
* ptol Distance from the plane.
Elements whose centroids are within distance ptol of the plane defined by the 3 points will be chosen. This allows you to choose planes of elements with an arbitrary orientation.

## Choose elements block

* x1, x2 x-range of the block.
* y1, y2 y-range of the block.
* z1, z2 z-range of the block.
Elements whose centroids are within the rectangular block defined by the 3 ranges are chosen. Note that the values given for one, two or all of the ranges can be identical and in that case, the block will collapse to a plane, line or point respectively.

## Choose elements block by layer

* x1, x2 x-range of the block.
* y1, y2 y-range of the block.
* z1, z2 z-range of the block.
* nlaybot, nlaytop Bottom and top element layer numbers.
Elements whose centroids are within the rectangular block which is defined by the 3 coordinate ranges, and which lie within the element layers defined by nlaybot and nlaytop are chosen. These layer numbers do not correspond to those given during grid generation but are simply defined by numbering each sheet of elements from 1 (bottom) to nz-1 (top) where nz is the number of sheets of nodes (2-D meshes) making up the grid.
This instruction is designed for grids that are regular in x and y, but which have a variable z for a given element layer, and can be used if the top and bottom elevations of a 3-D element layer vary spatially.
Note that the values given for one, two or all of the ranges can be identical and in that case, the block will collapse to a plane, line or point respectively.

## Choose elements by layer

* nlaybot,nlaytop Bottom and top element layer numbers.
Elements which are between the bottom and top layers are chosen.

## Choose elements top

All elements which are in the top layer are chosen.

## Choose elements list

* fname Name of the file which contains the list of element numbers.
The file consists of a list of element numbers, one entry per line. The procedure exits automatically when end-of-file is reached. The elements in the list are chosen.

## Choose elements xyz list

* fname Name of the file which contains the list of xyz-coordinates.
One set of coordinates should be entered per line. If a coordinate falls within an element, then that element is chosen. This instruction only works for rectangular structured meshes with the standard element numbering scheme, where the number of nodes in each of the x-, y-, z-directions (nx, ny, nz) is defined. It does not work for irregular grids generated with GridBuilder or GMS.

## Choose elements gb

* fname Name of the GRID BUILDER chosen elements file gb_prefix.echos.description.
* nsheet_bot,nsheet_top Bottom and top sheet numbers.
Elements flagged as .TRUE. in the file and that are between the bottom and top sheet are chosen.
The following instructions allow you to choose elements based on raster surfaces, as described in Section H. Since rasters are independent of the 3-D mesh, they do not have to be regenerated if the mesh changes, as is the case with Grid Builder or GMS formatted surfaces that have values corresponding to each node in the 2-D mesh.

## Choose elements between raster surfaces

* rasterfile Name of the raster file for the bottom surface.
* rasterfile Name of the raster file for the top surface.
The xy-coordinates of the element centroid are used to determine if it falls in a valid cell (i.e. a cell that has no missing values) in both raster files. If it does, the coordinates are used to interpolate a raster value from the cell’s four corner values. If the z−coordinate of the element centroid is greater than the raster value of the bottom surface, and less than the raster value of the top surface, then the element is chosen.
The following instructions use the same input data structure except that a single raster file is read and used for comparison:
          Choose elements above raster surface
          Choose elements below raster surface
The following instructions use the same input data structure except that there is an additional constraint that the element is chosen only if its current zone number is zero. They are intended to be used in conjunction with the instruction Assign zone zero:
          Choose elements between raster surfaces, iprop zero
          Choose elements above raster surface, iprop zero
          Choose elements below raster surface, iprop zero
The following instructions use the same input data structure except that GRID BUILDER nodal property files (i.e gb_prefix.nprop.description) are read instead of raster files. The element is chosen based on the z-coordinate of the first node in its node list:
          Choose elements between gb surfaces
          Choose elements above gb surface
          Choose elements below gb surface
The following instructions use the same input data structure except that GMS scalar files are read instead of raster files. The element is chosen based on the z-coordinate of the first node in its node list:
          Choose elements between gms surfaces
          Choose elements above gms surface
          Choose elements below gms surface

## Choose elements from arcview ascii thickness map

* arcfile Name of the Arcview ASCII file.
Reads an ARCVIEW ascii file containing thickness values (e.g. overburden thickness). For each 3-D element, its position (row and column) in the raster is determined and its centroid is calculated. It is chosen if its centroid is greater than the surface elevation minus the thickness at that location.

## Choose elements horizontal circle

* x_mid, y_mid, z_mid xy-coordinates of the centre of the circle and elevation of the circle.
* radius Radius of the circle.
* ptol Vertical tolerance.
* rtol Horizontal tolerance.
Elements whose centroids are within a vertical distance ptol of elevation z_mid, and within a horizontal distance rtol of the circle with centre x_mid, y_mid and radius radius are chosen. This allows you to choose elements in a domain that has a circular ground-plan.