Tracker

$images\btn_tracker.gif$ The tracker tool provides interactive distance and area measurements. To use the tracker, click on any start location. As the mouse is moved, the status bar will provide a distance from the starting point to the current position of the cursor.

If we click the tracker at several locations the distance reported will be through the polyline distance from the starting point to the current cursor location. Right click or press the ESC key to clear the tracker so it may be clicked at a new starting point.

$images\sc_tracker_01.gif$

For example, if we wish to know the distance from the Oakland Estuary around Treasure Island, back to the City front and then up to Sausalito we can begin by clicking the tracker cursor just outside the Estuary. We can then click a few more times to form a line going around Treasure Island and back to the City. At all times the status bar measurement readout will report the distance through the resultant line from the starting point to the current position of the cursor.

$images\sc_tracker_02.gif$ When we move the cursor up towards Sausalito we can see the distance over the indicated path is about 21671 meters.

To measure exact distances between specific locations, we will often use Snap together with the tracker tool to snap to an exact location, such as a point. Use the TAB key to jump the cursor to the nearest snap location as specified by the current Snap To modes.

By default, the tracker tool will report distances using the unit of measure used in the native coordinate system (projection) of the drawing. Pressing CTRL will shift the unit of measure to the units set in the Tools - Options dialogs. When using CTRL, distances will be computed over the Earth ellipsoid in use and reported in linear measurement units even if the drawing has not been projected.

Autoscroll

The tracker tool is a "live" cursor that is attached to the mouse cursor. Moving the cursor out of the active window after the first tracker click will cause an autoscroll. To avoid unintended autoscroll, remember to use ESC or a right click to end tracker scrolling before moving the mouse cursor out of the active window.

Press CTRL to Change Units and Computation

Press and hold down the CTRL key at any time to switch the tracker read out between native units and the default measurement unit specified in Tools - Options.

For example, in maps using Latitude / Longitude projection the native units are degrees. If we would like to see tracker distance in miles or kilometers we would hold down the CTRL key. The tracker in CTRL mode will show measurements in either English (miles and feet) or metric (kilometers and meters) depending on the setting of the Use English measurement units option in Tools - Options. Small distances will automatically use the smaller units (feet or meters) with the display jumping to larger units (miles or kilometers) for larger distances. Very small values will be reported using millimeters or inches.

Using CTRL also shifts the formulae used within Manifold to compute distance. The default tracker measurement uses plane geometry on the assumption that the component is seen in whatever projection is desired. By default, the tracker tool reports simply Euclidean distance in whatever units define the native, planar coordinate system of the drawing. Pressing CTRL shifts the computation to a full ellipsoidal trigonometric computation of the arc distance of the geodetic curves involved for length or areas.

The difference may be seen sometimes when using metric units so that the default tracker might report a distance of 50017 meters while a CTRL tracker reports, for example, 49.92 kilometers. The difference between the two is the difference between an Euclidean computation on a plane and an ellipsoidal computation on the Earth ellipsoid in use.

Accuracy

Normally, if a projection is well chosen for the region of interest the difference between the two computations (using or not using CTRL) will not be that great. A large difference between the two computations indicates either an error in georegistration or the inappropriate use of a projection. For example, GIS newbies sometimes attempt to combine data from multiple UTM zones into a single UTM zone, applying the single UTM zone to geographic regions far beyond where that particular zone's projection will be accurate. In such cases, there will be a significant difference between the value reported in native units as a result of a planar Euclidean computation and the value reported in default measurement units as result of an ellipsoidal trigonometric computation.

For highest accuracy first make sure that the component is correctly georegistered and second make sure that the projection used for the image is one that is scale-invariant and centered exactly upon the region of interest.

Because no projection is perfect at representing the curved surface of the globe, when measuring very large regions of the Earth there is likely to be a significant difference between the ellipsoidal trigonometric computation done with CTRL and the Euclidean computation done by default, even if the projection is well chosen. In such cases the CTRL computation is the more accurate one.

At times the difference between Euclidean and ellipsoidal calculations will be great even over relatively small regions. For example, when using the tracker tool with Mercator projections (which do not preserve distances accurately away from the Equator) there will be great differences between the tracker and the CTRL-tracker measurement. This effect may be very noticeable when using image servers to provide linked images in Mercator projection. When making measurements in such cases use the CTRL-tracker numbers.

Measuring Perimeter and Area with SHIFT

The tracker may also be used to measure area. The tracker reports the area within the closed region formed by connecting the starting point to the current position of the mouse. Press the SHIFT key at any time when using the tracker and the area denoted by the tracker path so far will be closed and a computed area will be reported to the status bar.

The SHIFT and CTRL keys may be combined to show areas in different units.

Example

The illustrations below show a drawing with three points. The drawing is in unprojected Latitude / Longitude so the native units used are degrees. We have set Snap To Points and have clicked on the tracker.

$images\sc_tracker_03.gif$

We first click on the uppermost point. As the cursor moves near the middle point the snap function will snap the square secondary cursor onto that point and…

$images\sc_tracker_03a.gif$ … the status bar will report the length of that segment in kilometers. At the scale used kilometers are a better match than meters. We can change the default units between metric and English (miles) units in Tools - Options .

$images\sc_tracker_03b.gif$ $images\sc_tracker_03a.gif$ Pressing the CTRL key while operating the tracker will toggle the units shown between native units (degrees, in this case) and "real" units (miles or kilometers).

$images\sc_tracker_04.gif$

We can click to anchor that tracker segment on the middle point. Now, when we move the tracker cursor near the third point the snap function will snap / extend the tracker line to the third point.

$images\sc_tracker_04a.gif$ With the square cursor box on the third point the status bar will read the overall length of the tracker line, which now consists of two segments.

$images\sc_tracker_05.gif$

At any time we can press the SHIFT key to temporarily close the tracker line back to the originating point. In the above illustration, we've positioned the mouse cursor near the third point so that the tracker line snaps to the third point, but we have also pressed the SHIFT key so that the tracker line closes the figure back to the first point.

$images\sc_tracker_05a.gif$ The result in the status bar is the length of the entire tracker perimeter as well as the area in square kilometers.

Scroll Wheel

When using a mouse with a scroll wheel we can use the scroll wheel to zoom in and out. This works even in the middle of commands such as the tracker tool. By default the zoom will be centered on the mouse position. Holding the CTRL key while operating the scroll wheel will force the zoom to be near the center of the opened window.

Notes

Keep in mind that projected maps use linear measures such as feet or meters as their native units. Toggling the tracker tool using CTRL may therefore in some cases toggle between the native unit and "real" units that in both cases may be meters.

In addition to using a right click or the Esc key to cancel tracker function, the ALT- Tab key combination may also be pressed if some non-Manifold program or information dialog (such as a "download complete" notification when downloading files in background) has snatched context away from the Manifold window.

When used with layouts, the tracker tool will report in either English (inches) or metric units depending on the setting of the Use English measurement units option in Tools - Options.

Tracker computations depend upon correct georegistration of the component being measured. If the component has not been correctly imported and georegistered the CTRL computation cannot be accurately done.