====== How to create a CODA pelvis relative to anatomical landmarks ======

The [[visual3d:documentation:modeling:coordinate_system|segment coordinate system]] for the [[visual3d:documentation:modeling:segments:coda_pelvis|CODA pelvis]] is the mid-point between the ASIS markers. The anatomical landmarks are actually posterior to the ASIS markers because the motion capture system tracks the centre of the marker.

===== Correcting the CODA definition =====

This is a modest error, but it can be corrected by creating landmarks that are posterior to the ASIS markers by the radius of the motion capture marker.

==== Step 1. Confirm the subject metric containing the marker radius ====

If the metric doesn't exist, create a new metric. Switch to **Subject Data/Metrics** TAB.

{{:SegmentExample4_1.png}}

Select **Add New Item** and enter in the name **Marker_Radius** and the value.

{{:SegmentExample4_2.png}}

In this example, the marker has a diameter of 1.2 cm. Visual3D uses [[visual3d:documentation:pipeline:model_based_data_commands:units_of_measurement|MKS units]], so the value is 0.006 meters. **NOTE**: If the radius is not consistent with your marker radius, modify the value.

==== Step 2. Create landmarks for the ASIS markers ====

{{:SegmentExample4_4.png}} {{:SegmentExample4_5.png}}

**NOTE**: the Medial-Lateral (ML) Edit box contains the [[visual3d:documentation:pipeline:expressions:expressions_overview|expression]] **-MARKER_RADIUS** to define the offset.

==== Step 3. Create a CODA Pelvis ====

Follow the usual steps to create a CODA pelvis segment.

{{:SegmentExample4_6.png}}

In this example, the locations defining the segment coordinate system for the pelvis are the landmarks **RT_ASIS** and **LT_ASIS**, and the marker **SACR** located at the midpoint of the PSIS markers.

The tracking markers are **RASI**, **LASI**, and **SACR**.

{{:SegmentExample4_7.png}}