PlanetGIS User Guide/The Display Manager
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The Display Manager
Introduction
The display manager is the primary interface for the user to change the appearance of a map.
The main functions of the display manager are:
- Adding displays, that is to say creating displays from references to feature classes of the current map or an external map, or creating groups to contain displays;
- Toggling the visibility of displays;
- Changing the order of displays;
- Altering the properties of a display.
Adding displays
Creating display groups make the map more manageable, especially for toggling the visibility of a group of related displays.
Toggling visibility
The visibility of a display is influenced by quite a few factors. Conditions for a minimum and maximum scale of display may be set in two places: the display's Visibility tab and for each draw element of the selected style. If there are any restrictions based on scale, a little yellow question mark will be displayed with the “eye” (
). If you click on an eye and the visibility could not be toggled due to a scale restriction, you will get a message asking if you'd like to go to the appropriate place to change the setting.
Drawing order
The order in which displays appear in the display manager originally determined the order in which feature classes and theme layers where drawn. In this sense the concept of a display is closest to a layer as used in transparent overlays. Recent versions of PlanetGIS introduced “smart draw ordering”, style element prioritization and label masking.
Smart draw ordering
Smart draw ordering is an option that is on by default and may be disabled in a view's properties (See Working With Views). When disabled, the topmost display in the display manager is considered first when a map is being rendered and the bottommost display is considered last. We use the word “considered” here because style element prioritization may still change the order in which displays are processed.
With smart draw ordering enabled (the default), raster images will always be rendered first, then the fills of areas, then lines and outlines, then symbols and lastly text and labels. Within the same element type (e.g. raster images) the order of the displays determine the order of rendition, i.e. the first display referencing raster images is draw before the next.
Style element prioritization
Individual elements of a style may be assigned a priority number between -10 and 10. Elements with a priority of -10 will all be drawn first (in display order) and elements with a priority of +10 will all be drawn last. A zero priority (the default) causes element to be drawn in their natural order, as determined by the smart draw ordering setting.
Label masking
Automatic labels are placed in the final step of map rendition, once all the labels have been collected during map rendering. A display may however be set to “draw after labels” which has the effect of masking out underlying labels (and other features) if that is desired. Any such displays are drawn as the very last step in map rendition (although before floating objects).
Changing the order of displays
To change the order in the list of displays, you can drag a display to a new position. When doing this, please be aware that grouped displays will accept a dragged-and-dropped display as a member of the group. Dropping directly onto the name of a node suggests that you want to place your selected items inside that node. If you are only changing the order of displays, be sure to drop a display slightly to the right of the name of the target.
Display Properties
General properties
Feature class: Here you can view or change the feature class (or thematic layer) that it is associated with the display.
Style: View or select a style to use when rendering the feature class on the map.
Draw after labels: Mark this display to be rendered after labels have been placed. This is useful to create a “mask” to hide certain parts of a map, including labels.
 | Note: | A feature class may be from a different project than that which the display belongs to. The style may also be selected from an external project, and will initially be set to the feature class' default style (which will be in the same project as the feature class). It might be a good idea to select a “local” style for the display, so that an external project doesn't get changed inadvertently when modifications are made to the style to suit the current project. (The external project might also be used in other projects). The style selected for a display must be either belonging to the display's project, or the feature class' project. It cannot be from yet another project. |
Visibility
Scale conditional display. It is very useful to be able to display certain things only once a user has zoomed in close enough. For example, displaying residential roads at a smaller scale than 1:500000 may clutter a map; on the other hand displaying the name of a country or a point for a city at a scale greater than 1:50000 could also be undesirable.
| Note: | A map is “large scale” when features are shown in more detail because the scale fraction, e.g. 1/10000 is greater than the scale fraction of a “small scale” map, e.g. 1/500000. A “small scale” map indeed has a much larger spatial extent than a “large scale” map. The most recent version of Planet uses the words “zoomed in closer than or at” and “zoomed out further than” to avoid any confusion. When the zoom in/out metaphor is used, the scale-conditions may be visualized as at certain altitudes of the observer. The denominator of the scale fraction (the amount that is entered into Planet) can then be seen as a measure of the altitude of the observer. The first condition (zoomed in closer than or at) is the “upper bound” and the second condition the “lower bound” for scale-conditional display. The upper bound is inclusive while the lower bound is not. That means that if the scale of the map is exactly the same as the upper bound the display will be rendered but if the scale of the map is equal to the the lower bound (zoomed out further than) it will not be rendered. This is important especially with styles where several drawing elements might be “passing the baton” at different scales. |
Maximum feature density. While the usage of map scale is useful to avoid displaying too much information, it does not take into account localized concentrations of features. For example, when choosing an “upper bound” scale for the display of minor roads, one value will be suitable for metropolitan areas while it will be unsuitable for rural areas. When panning away from a metropolitan area into a rural area (keeping the scale constant), the map will suddenly become too sparse with roads being hidden even though they are important enough to show. To overcome this problem, one can specify a fairly generous “upper bound” scale and further limit the visibility of a display to a maximum density of features. Feature density is calculated by counting the features that will be wholly or partially displayed on a particular map and dividing it by the area of display, measured in square centimetres or square inches. Click on the calculate button to see the feature density of the current map view. You can use that as a guide for setting up your limit.
| Note: | It may be more convenient to apply scale conditions to style elements (See Working With Styles) when setting up more than one view of a map. However, the maximum feature density limitation is only available for a display. |
Options
- Tagged: yellow tags will not be placed for selected features
- Selected: features shown in this display cannot be selected at all (tagging will also not be possible)
- Edit: editing of feature coordinates will not be allowed
- Located: the display will not appear in the locate by key/label/attribute functions.
Labels
When using automatic placement the criteria for finding optimal locations for labels are as follows:
- For point features with a symbol element in the selected style the preferred location for a label is the upper-left corner of its symbol. For point features without symbols (only a font) the text will be aligned to the point, but according to the alignment settings of the font. (See Working with Styles).
- For line features a trade-off is found between the longest segment and the most horizontal segment. The horizontal alignment setting of the style's font is ignored, but a vertical alignment of any value other than “center” will place the label either above or below the line feature (above has slight preference).
- For area features the preferred location is at the centroid of the polygon but labels will be placed anywhere inside the area including crossing a boundary by up to 1/3 of the length of the label.
Use fixed placement to place labels at exact locations as follows:
- Point features with symbols: always at upper-left of symbol. Point features without symbols: centred on point but aligned according to font settings in style.
- Line features: exactly on the mid point of the line as determined by half of the total distance along the line.
- Area features: exactly at the centroid. If the centroid falls outside the area, the location of the label is moved to the point along the boundary that is closest to the centroid.
Raster images cannot be labelled.
Label entities with. The first option (“Label”) is the internally stored label of the feature in its entirety. Up to 3 different labels can be stored by concatenating them, separated by the pipe character (|). You will have to select “Label #1/2/3” to indicate which part of the label is to be used. This is useful to label roads by both their street name as well as route number and use a different styling for route numbers. An example of such a label would be “Baker Street|M17”. After the internal label fields, fields from attribute tables attached to the feature class are shown.
 | Tip: | Labelling with attributes can be slow at first because the labels have to be fetched from a database and kept in memory. The advantage is that the labels are more “dynamic” although you have to explicitly reload labels before changes will be visible. Labels can be reloaded by pressing <Ctrl> while clicking the Redraw button, or with the Tools->Database->Refresh tables menu command. |
Allow overlap. Select this option to display all labels at their optimal locations, regardless of overlaps. If not selected, many labels might be dropped. This option labels large numbers of features much quicker than the automatic labelling algorithm.
Draw symbol only if label can be drawn. (Only applicable when allow overlap is unchecked). If selected, labelled point features will not be displayed if the label cannot be placed due to congestion. This is helpful because labelled points are themselves “no-go zones” for labels to avoid ambiguities. When points are dropped in highly congested areas, space is freed up to place labels for other points. This becomes especially useful when priorities are assigned to labels so that less important points can be dropped to allow important points to be labelled instead.
| Tip: | Sometimes a point is not getting labelled for no apparent reason. If this happens, check for a duplicate point or one in very close proximity. A label will not be automatically placed if it is too close to another labelled point. Also check that a point feature doesn't contain multiple coordinates in close proximity. Another common reason for labels not getting placed is the absence of a font in the selected style (See Working with Styles). |
Show duplicates. The default behaviour is not to place the same label text in close proximity to another. Check this option to see duplicates.
Orient to feature. (Applicable to line features only). The default behaviour is to align line features' labels with the straightest available section. Uncheck this to have horizontally aligned labels, e.g. for route numbers.
Label priority. Increase this number to a maximum of 10 to indicate the relative importance of labels. The priority number is used to assign a higher “penalty” for dropping a label in a congested area. The labelling algorithm attempts to find the lowest score overall.
Tags
Open. (Enabled only if special fields have been set up. See Linking to Information). Select a special field to execute the special field’s defined action when a feature is selected (tagged). A special field action may be to open an image, document or launch a web hyperlink.
Execute. Choose a registered plugin module or supply the path to a Windows application which will be launched with a feature identifier as a parameter when a feature is selected (tagged). Custom software modules can be linked in this way to provide specialized functionality to Planet.
Tag fields. By default, the text displayed in a feature's tag is the feature's unique identifier (GID or key.) A display may optionally be set up so that additional information will be displayed when features are tagged. This may be the feature's label or attributes. You can select more than one entry, e.g. the key to appear on the first line and some database information on subsequent lines in a tag.
Show field names. Select to display the names of attributes (field names) in the tag. This may help to explain the meaning of text appearing in a tag.
Legend
Show in legend. Uncheck to remove this display from the automatically generated legend.
Legend label. You may provide an alternate description to appear in the legend (instead of the display's name).
Display group members. You can uncheck this for grouped displays so that only the group display is shown in the legend (not the contained displays).
Column break. Check this to force a column break in the legend. If a legend is wide enough, Planet will create two or more columns to display all the items.
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