Landmarks are like virtual markers, whose positions are computed automatically by Visual3D based on the positions of the actual markers, segments, or equations, according to rules which you define. Landmarks are created by either specifying a location in a segment coordinate system or by transforming markers or other landmarks into a significant location. These landmarks are like virtual markers, whose positions are computed automatically by Visual3D based on the positions of the actual markers, segments, or equations, according to rules which you define.

The landmark mechanism is the key to Visual3D’s highly flexible modeling capabilities. A single example suffices to suggest the usefulness of markers. Frequently, target markers are mounted on short posts which protrude away from the skin surface, and consequently, defining a segment using medial and lateral markers as described above will tend to over-estimate the segment’s radii. You can eliminate this problem by creating landmarks which are like “shadow versions” of the actual markers, moved inward by the length of the marker posts to better approximate the skin surface, and then define your segments based on these landmarks.

Landmarks can be 'calibration only' or can be calculated for the motion files.

1. Calibration Only: These landmarks are used only for segment calibration, but are not needed (or produced) for any associated motion file(s).
2. Generated for Assigned Motion Files: These landmarks are generated for both the calibration file and the associated motion files. These landmarks may be used for tracking a segment.

The landmark dialog can be broken into 3 sections:

1. Define Orientation
2. Offset
3. Calibration Only

Landmarks can be defined relative to a target, along a line, on a plane, or as a projection onto a line or a plane. Landmarks can be defined in lab space or in a segment coordinate system.

Any type of landmark can be defined using offsets in three directions, a user has the option to define the offsets manually (X/Y/Z or ML/AP/AXIAL) or they can use the “Offset to Existing Calibration Target or Landmark” option.

The Offset to Existing Calibration Target or Landmark option allows users to get the location of a target (or landmark) relative to a plane or segment coordinate system. The offsets are populated automatically. This option can be used any time all three offsets can be specified, but is described as a special type for Type 4 since this is a crucial example.

Offsets should be specified in meters unless “Offset by Percent” is checked. It is possible to specify offsets as Expressions

When a landmark is set to “Calibration Only” it means that the landmark will only exist in the calibration file. When this option is not checked, it means that the landmark will be created in the associated motion files. If a landmark is created in the associated motion files, it can be used to track a segment or for other calculations.

Although there are many different ways to define a landmark, all of these methods result in a “virtual marker” whose position is calculated by Visual3D throughout motion trials. These landmark types include:

1. as a point relative to one marker/landmark
2. as a point on a Line
3. as a point in a Plane
4. as a point Projected on a Line
5. as a point Projected on a Plane
6. as offsets (using regression equations or relative to a segment)
7. relative to three points
8. digitized with a pointer
9. as a functional point

See the Landmark Types page for a detailed explanation of each definition type.

Digitized landmarks are defined using a digitized pointer and again result in a “virtual marker” being computed by Visual3D relative to the positions of other, existing markers. The goal of digitized landmarks is to allow users to track anatomical landmarks without having to place markers on that location. See instructions for using a digitizing pointer to learn more.

Functional landmarks indicate the locations of functional joints. Calculating functional joint positions requires movement of one segment relative to another segment. The chosen algorithm then searches for a point (or for a one degree of freedom joint, an axis) that is stationary relative to the 2 segments (or 2 sets of markers).

Visual3D's algorithms for computing functional joint positions are adapted from:

• Schwartz MH, Rozumalski A (2005) A new method for estimating joint parameters from motion data. Journal of Biomechanics, 38, 107-116; and
• Jensen E, Lugade V, Crenshaw J, Miller E, Kaufman K (2016) A principal component analysis approach to correcting the knee flexion axis during gait. Journal of Biomechanics, in press

See the in-depth explanation of functional joints to learn more.