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|Don't use a toggle between two levels of zoom because your only convenient interaction does not support steps or graduations. Zooming via double-tap, for example, has value in some cases but is not a typical zoom and does not cover the majority of the needs for zooming into data.
Methods to zoom in and out, especially suggested icons, ways of indicating zoom level (national to house) and literal scale axes and bars. Try to come up with good examples that are not maps. Charts of financial data, again.
Data in dense information arrays, such as charts, graphs and maps, must be able to change the level of detail presented by a zooming function or metaphor.
High density information, such as that in charts, graphs and maps, can be especially difficult to use on small screen devices. Showing a sense of the whole space can preclude viewing sufficient details for analysis and other uses.
Zooming into (and out of) the information is the general solution. As this is similar to changing the depth of view, it is a different axis than scrolling, so requires a unique control set.
Coordinated with changing the zoom level is communicating a sense of scale, whether relative or absolute.
On-screen buttons - plus/minus buttons (or +/- in a magnifier)
Interactive scale - A bar may be provided for direct control, that indicates the whole range of zoom; generally, icons are used at the top and bottom of the control that indicate the range of the control, such as (for a map) the country for the maximum zoom out, and a house to indicate block-level for zooming in. These controls may support direct selection of a zoom level, or direct control of the zoom level by moving the slider dynamically. They are also often combined with on-screen buttons to offer all options.
Screen gestures - The most common of these is the . Other on-screen gestures are also used, including single-finger spinning actions (clockwise to zoom in, counter-clockwise to zoom out). These may be useful for systems that cannot (for technical or user needs) support multi-touch.
Hardware buttons - devices that do this a lot, like GPS, might have dedicated zoom buttons... also, can repurpose unused hardware keys; if the 5-way pad is used to scroll along the x/y axes (and select points) the volume rocker can be repurposed to control the z-axis and be a zoom control.
Additional methods may emerge in the future, such as the use of sensors to change detail level as the device is moved towards the viewer's eye.
Some or all of these may be combined, for interfaces that work on multiple devices without change, to appeal to different user types, or to surmount lack of affordance in screen gestures.
For controls that do not integrate a scale, some sense of scale must be provided. This may be an explicit scale (labeling axes or a bar of distances) or implicit, by showing items of a well-understood size.
talked about above mostly???
must take effect immediately...
Display the zoom level WHILE zooming at least... gesture and hardware keys, as well as on-screen controls, must display the new zoom rate momentarily while the zoom is happening, so they
use standard units of measure, SEE THE OTHER PATTERN ABOUT THIS ONE...
Don't use a toggle between two levels of zoom because your only convenient interaction does not support steps or graduations. Zooming via double-tap, for example, has value in some cases but is not a typical zoom and does not cover the majority of the needs for zooming into data.