# HydroGeoSphere/Selecting Nodes

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

## Clear chosen nodes

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

## Choose nodes all

All nodes in the domain will be chosen. This is useful if you wish to assign a property to all nodes in the grid. For example, you could issue this instruction and then assign a uniform initial head for the problem.

## Choose node

* x1, y1, z1 xyz-coordinate.
The node closest to the given coordinate will be chosen.

## Choose node number

* i Node number.
The node with this number will be chosen. You should use this instruction with caution since node numbering will change if the grid structure changes.

## Choose nodes x plane

* x1 x-coordinate of the plane.
* ptol Distance from the plane.
Nodes within distance ptol of the plane defined by the equation x = x1 will be chosen. This command is particularly useful when assigning boundary conditions to a specific side of a rectangular domain.

## Choose nodes y plane

As above but for the y-plane.

## Choose nodes z plane

As above but for the z-plane.

## Choose nodes 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.
Nodes within distance ptol of the plane defined by the 3 points will be chosen. This allows you to choose planes of nodes with an arbitrary orientation.

## Choose nodes 3pt plane bounded

* 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.
* x4, x5 x-range of the block.
* y4, y5 y-range of the block.
* z4, z5 z-range of the block.
Nodes within distance ptol of the plane defined by the 3 points and within the rectangular block defined by the 3 ranges will be chosen.

## Choose nodes block

* x1, x2 x-range of the block.
* y1, y2 y-range of the block.
* z1, z2 z-range of the block.
Nodes 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 nodes top

All nodes in the top sheet of the domain are chosen.

## Choose nodes top block

* x1, x2 x-range of the block.
* y1, y2 y-range of the block.
* z1, z2 z-range of the block.
Nodes in the top sheet 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 nodes bottom

All nodes in the bottom sheet of the domain are chosen.

## Choose nodes sheet

* nsheet Sheet number.
All nodes in this sheet are chosen.

## Choose nodes gb

* fname Name of the GRID BUILDER chosen nodes file gb_prefix.nchos.description.
* nsheet_bot,nsheet_top Bottom and top sheet numbers.
Nodes flagged as .TRUE. in the file and that are between the bottom and top sheet (inclusive) are chosen.

## Choose nodes list

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

## Choose nodes xyz list

* fname Name of the file which contains the list of xyz-coordinates.
The file consists of a list of xyz-coordinates, one entry per line. The node nearest the given coordinate will be chosen. The procedure exits automatically when end-of-file is reached.

## Choose surface flow nodes

Nodes currently flagged as surface flow nodes are chosen.

## Choose nodes top boundary

Nodes around the top edge of the surface flow domain are chosen.

## Choose nodes top overlay file

* fname Name of the GRID BUILDER overlay file.
Only the first group of entries in the overlay file are considered. If an overlay line segment intersects an element edge, then the node in the top sheet closest to the point of intersection is chosen.

## Choose nodes between gb surfaces

* surffile Name of the GRID BUILDER nodal properties file gb_prefix.nprop.description for the bottom surface.
* surffile Name of the GRID BUILDER nodal properties file gb_prefix.nprop.description for the top surface.
For the 3-D mesh node with the same x− and y−coordinate as a 2-D mesh node, if the z−coordinate of the 3-D node is greater than the nodal property value given in the bottom surface file and less than the nodal property value given in the top surface file the 3-D node is chosen.

## Choose nodes tecplot geometry

* geofile Name of the TECPLOT geometry file which contains polyline data.
The node nearest to each point on the polyline is chosen.

## Choose nodes 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.
Nodes 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 nodes in a domain that has a circular ground-plan.

## Choose nodes active/inactive boundary

Nodes which lie on the boundary between active and inactive elements will be chosen.
This instruction was intended to be used to select the surface of nodes on top of the set of active elements so that flow boundary conditions (e.g. head equals elevation) could be assigned.

## Write chosen nodes

* fname Name of the file to which the chosen node numbers will be written.

## Write chosen nodes xyz

* fname Name of the file to which the chosen node information will be written.
As above except xyz-coordinates are written instead of node numbers.