GPS Manager User Manual -- Groups

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## Groups

Groups are very useful in cataloguing the available data and in selecting information to be displayed on or cleared from the map, or to be transferred to/from the receiver or to/from a file. Groups are also used by GPSMan to present the results of a search as described below and to perform some operations on sets of waypoints (defining an average waypoint, changing the position formats and computing clusters of waypoints).

A group contains a certain number of data items and is represented internally as set of item names (together with their types). Operations on a group may fail or succeed only partially if one of its elements is not currently in the data-base.

Groups can have other groups as elements but one group cannot be an element of itself even if indirectly (in technical terms: groups are well-founded sets).

Clearing from the map an item that belongs to a group that has been displayed will not affect the display-state of the group. To be sure that all the elements of a group are actually displayed, the user should clear the group from the map and then display it again.

Deleting from or adding items to a group will not affect their display-state.

Forgetting a group will delete permanently the group from the data-base but not its elements. This operation is not prevented by the fact that any of its elements cannot be cleared from the map.

Saving a group (to a GPSMan file) will save all the information on the group and on its elements.

Creating an average waypoint from the waypoints in a group can be made from the group window. The coordinates of the new waypoint will be the averages of the latitudes, longitudes and altitudes of waypoints in the group and its sub-groups (recursively).

Converting the position formats of the waypoints in a group can also be made from the group window. All the waypoints in the group and its sub-groups (recursively) will change to the (same) selected position format. If one of the waypoints is being edited, the position format in the edit window will also be changed and the position will revert to its initial value (when the edit window was created).

Clusters of waypoints can be created from a group by taking the waypoints in it (and its sub-groups, recursively) as centres of the clusters and searching the data-base for waypoints that fulfil a selected condition for each centre. The conditions that can be tested are: that the waypoint is within a given distance range of the centre, or that the waypoint belongs to the quadrangle of given latitude and longitude ranges whose middle-point is the centre. It is obvious that the first condition will be much slower to evaluate than the second, and therefore making clusters based on quadrangles should be preferred when the number of waypoints currently defined is large. Each cluster will be created as a group: its name is of the form `Cluster` n, and its remark has the name of the centre and either the quadrangle dimensions, or the distance range.

Input/output operations on the elements of a group allow for selecting which items of which types to read or write. In general the user will choose the groups and the item types for the operation. Then GPSMan collects in a list the names of the items of the given types that belong to the selected groups. This list of names is used to perform the I/O operation.

Selecting the "Group" type means that the search for items will be done in the groups that are elements of the selected groups, recursively. In more technical terms, the resulting list may be seen as a flattening of the group structure. In no case the list of names will contain names of groups.

Details of each specific operation are as follows:

- in output operations, the "All" menu entry means that all groups will be considered. When saving or putting elements this also means that all types should be considered.
- in input operations, there is the option of reading either the items whose names are not in the list of names GPSMan builds, or the items having the names in that list. The former is useful for preserving data in the selected groups; items that are not in the data-base will also be read in. The latter is useful for updating or restoring the information in the selected groups without affecting the other data; items that are not in the data-base will not be read in.
- when exporting or importing information to/in GPStrans format, a single type (apart from "Group") must be chosen because of the structure of GPStrans files.
- when getting information from the receiver the "Track" type cannot be used as there is no point in updating or changing previously recorded tracks.

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