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Display vector arrows

Vector arrows can be displayed for point themes where the attribute table includes attributes defining length and direction of a vector or the vector components. For projected views (requires source data in decimal degrees), the direction is the compass direction from each point, and the vector components are along the north and east direction from each point. If the source data are projected, the vector direction and components must be in the X-Y co-ordinate system of the projected source data. It is required that both the Map units and the Distance units are defined in the View properties, as the map units define the actual length of a vector, while the display units define the corresponding display length of the vector.

The display of vector arrows involve the following steps:

1. Make the requested theme(s) active

2. Specify line width of the arrow

3. Select type of vector definition

4. Select the proper attribute fields

5. Specify maximum display length of arrows

6. Specify arrow scale

7. Select arrow direction

8. Create arrows

These steps are described in more detail in the following:

Make the requested theme(s) active

The theme(s) for which to display vector arrows must be active, and they must all be point themes comprising attributes defining the vectors. Also, as the arrow colours are taken from the legend of each theme (see Step 6), each legend should be classified in the way the user wants the arrows to appear.

Specify line width of the arrow

Only one type of arrow is currently available, and this is a solid-line arrow, where the arrowhead is drawn at 45 degree angles and extending 0.25 of the size of the vector. The user is prompted to specify the line width, in millimetres, of the arrow.

Note:

The user's choice of arrow line width will be used for all active themes. If the user wants to vary the line width for different themes, only the themes with the same line width should be made active and then processed at a time.

Select type of vector definition

For each active theme, the user is prompted to select how the vectors are defined. The choices are:

INSROP00090000.gif Length/Speed and direction

INSROP00090000.gif X/Y (E/N) components

Select the proper attribute fields

Depending on the choice made in the previous step, the user is presented with a list of attribute fields within the current active theme and prompted to select the required vector definition attribute fields (one at a time).

Specify maximum display length of arrows

The user is prompted for how long the longest arrow should be when displayed in the view. The figure is in distance units. If e.g. the measurement points are stored in a 100-km mesh, it might be advantageous not to let the longest arrow be longer than 100 km.

Specify arrow scale

To establish the display length of the arrows, the display scale of the arrows must be known. For the current active theme, the user is prompted to specify how many vector units are to be represented by 1 distance unit. If the distance units are km, and the user specifies 2, this means that a vector with length 10 (of units used in the theme attribute table) will be displayed as a 5-km long arrow. The user should select a scale that fits the display scale, the distance between the data points, and the maximum vector length.

The default value is calculated from the specified maximum display length and the maximum vector length (as derived from the data in the attribute table). Entering a smaller number than the default value means that the longest arrow will be longer than the specified maximum display length and vice versa.

Select arrow direction

For some data (e.g. wind data) the direction specified by the vector data may be the from direction and not the to direction. As default the arrows will point in the to direction, but the user is prompted to answer Yes or No whether the arrows should be pointing in the reverse direction.

Create arrows

To create an arrow, the location, length, direction, colour and width of the arrow must be specified. The location is determined from the co-ordinates of the selected points in each active theme. If no points in a theme are selected, all points in this theme are used. The display length is taken from the vector attribute(s) and the user-specified arrow scale. The direction is determined from the vector attribute(s), and for project views the direction is in the X-Y co-ordinate system specified by the local North-East co-ordinate system in each point. The width of the arrow was specified by the user in the previous step.

The colour of an arrow is taken from the legend of the theme. This may seem as an unnecessary approach, as it may appear easier to automatically update the legend based on the vector length. However, as different users may want to classify the arrows differently, we have not found it beneficial to use this latter approach. Taking the colours from the legend classification also means that the point theme can be classified by any attribute, not just the arrow length. This may be quite useful for some purposes. For instance, if a point theme include both ice concentration and ice drift vectors, the vectors may be displayed based on ice drift speed and with colours based on classified ice concentration values. Also, vector arrows can be used to show migration routes and with colours showing number of migrating species.

To display vector arrows it is not necessary that the active theme(s) be displayed in the view (switched on). If the themes are displayed, both the legend symbols and the vector arrows will be displayed. To only display arrows, switch the theme(s) off. The vector arrows will anyway be displayed in the viewframe in a layout, but to include the theme legend in the legendframe in a layout, the theme must be displayed in the view.