Field Coordinate System page - add image descriptions and other readability fixes

docs/source/game_specific_resources/field_coordinate_system/field-coordinate-system.rst

@@ -1,28 +1,45 @@
 .. _first field coordinate system:
 
-*FIRST* Tech Challenge Field “Coordinate System" Definition
-===========================================================
+*FIRST* Tech Challenge Field Coordinate System Definition
+=========================================================
+
+.. meta::
+   :description: This document defines the FIRST Tech Challenge Field Coordinate System which can be used to specify position on the playing field.
+
+Summary: The *FIRST* Tech Challenge Field Coordinate System is a Cartesian Coordinate System of three dimensions.
+The X and Y axes will refer to a position on the field and the Z axis a height above the field.
 
 Scope
 -----
-
-This document defines the “standard” Coordinate System (orthogonal axes)
-definition for a *FIRST* Tech Challenge playing field. This definition can be
+  
+This document defines the Field Coordinate System 
+for a *FIRST* Tech Challenge playing field. This definition can be
 used for consistent field-centric navigation, target localization and path
 planning.
 
-Reference frame
+Reference Frame
 ---------------
 
 The reference frame for this definition is the field perimeter wall, adjacent
-to the RED Alliance Station (known here as the: RED WALL).  The definition is
+to the red Alliance Area, known here after as the Red Wall.  The definition is
 from the perspective of a person, standing outside the field, in the center of
-RED WALL, looking towards the center of the field.
-
-Caveat: If the Red Alliance Station is ever adjacent to two perimeter walls,
-the RED WALL will be the one with *most* contact with the Alliance Station. If
-the red Alliance Station is ever adjacent to two perimeter walls EQUALLY, then
-the most clockwise of the two walls will be considered to be the RED WALL.
+Red Wall, looking towards the center of the field.
+
+.. note:: 
+   If the red Alliance Area is ever adjacent to two perimeter walls,
+   the Red Wall will be the one with *most* contact with the Alliance Area. If
+   the red Alliance Area is ever adjacent to two perimeter walls *equally*, then
+   the most clockwise of the two walls will be considered to be the Red Wall.
+
+Coordinate System
+-----------------
+
+The Field Coordinate System is a Cartesian Coordinate System of three dimensions.
+X and Y will refer to a position on the field.
+Z will refer to a height above the field.
+You may use any length measure as long as the same measure is used for all three axes.
+The coordinates are ordered (X, Y, Z).
+Example: coordinate position (10, -10, 0) has X = 10, Y = -10 and Z = 0.
 
 Origin
 ^^^^^^
@@ -35,85 +52,131 @@ mat.
 X Axis
 ^^^^^^
 
-Looking at the origin from the RED WALL, the X axis extends through the origin
-point and runs to the right and left, parallel with the RED WALL. The X axis
+Looking at the origin from the Red Wall, the X axis extends through the origin
+point and runs to the right and left, parallel with the Red Wall. The X axis
 values increase to the right.
 
 Y Axis
 ^^^^^^
 
-Looking at the origin from the RED WALL, the Y axis extends through the origin
-point and runs out and in, perpendicular to the RED WALL.  Increasing Y values
-run out (away) from the RED WALL.
+Looking at the origin from the Red Wall, the Y axis extends through the origin
+point and runs out and in, perpendicular to the Red Wall. Increasing Y values
+run out (away) from the Red Wall.
 
 Z Axis
 ^^^^^^
 
-Looking at the origin from the RED WALL, the Z axis extends through the origin
+Looking at the origin from the Red Wall, the Z axis extends through the origin
 point and runs up and down in a vertical line. Increasing Z values extend
 upwards.
 
-Rotation about Axes
+Rotation About Axes
 ^^^^^^^^^^^^^^^^^^^
 
 When considering rotations about an axis, consider yourself looking down the
-(positive) axis of rotation from the positive towards the origin. Positive
-rotations are then CCW, and negative rotations CW.
-
+axis from the positive end towards the origin. Positive
+rotations are then counterclockwise and negative rotations clockwise.
+   
 .. figure:: images/image1.jpg
-   :width: 35%
-   :align: center
-   :alt: Coordinate Axes
-
-   Figure 1: Coordinate Axes
+   :alt: X, Y and Z coordinate axes.
+
+   Counterclockwise rotations about each axis
+
+   Imagine looking down the positive Z axis towards the origin.
+   This would be like standing in the middle of the field looking down.
+   A positive rotation about the Z axis would be counterclockwise.
 
-An example: consider looking down the positive Z axis towards the origin. This
-would be like standing in the middle of the field, looking down. A positive
-rotation about Z (i.e. a rotation parallel to the X-Y plane) is then CCW, as
-one would normally expect from the usual classic 2D geometry.
+Example: a robot spinning clockwise on the Field is making a negative rotation about the Z axis.
 
-Examples
---------
+Field Configuration Examples
+----------------------------
 
-Below are two examples illustrating this Axes definition.
+Below are two examples illustrating the Field Coordinate System for different
+*FIRST* Tech Challenge field configurations.
 
 .. note::
-   Note that in both cases the Red Alliance members are facing out,
-   along the positive Y axis.
-
-   However, in the “Diamond” field configuration, the X axis is pointing
-   towards the Blue Alliance, but in the “Square” field configuration
-   the Y axis is pointing towards the Blue Alliance.
-
-
-.. figure:: images/image2.jpg
-   :width: 75%
-   :align: center
-   :alt: RES-Q
-
-   Figure 2: FIRST Tech Challenge RES-Q game field orientation
-
-.. figure:: images/image3.jpg
-   :width: 75%
-   :align: center
-   :alt: Cascade Effect
-
-   Figure 3: FIRST Tech Challenge Cascade Effect game field orientation
+   In both field configurations the red Alliance is facing out along the positive Y axis,
+   and the Z axis points up from the center of the field.
+
+Diamond Field
+^^^^^^^^^^^^^
+
+.. figure:: images/first-res-q-field.png
+   :alt: A diamond field with X, Y and Z axes shown.
+
+   The FIRST RES-Q game field
+
+   In a diamond field configuration the two Alliance walls are adjacent.
+   The field is rotated 45 degrees such that both Alliances face the audience.
+   From the audience perspective the field forms a diamond shape.
+   The Red Wall will be on the right as seen from the audience.
+   The Y axis points across the field as seen from the Red Wall. 
+   The X axis points to the Blue Alliance.
+
+Square Field
+^^^^^^^^^^^^
+
+.. figure:: images/into-the-deep-field.png
+   :alt: A square field with X, Y and Z axes shown.
+
+   The Into The Deep game field
+
+   In a square field configuration the two Alliances face each other across the field.
+   The field is oriented such that the Red Wall is on the right as seen from the audience.
+   The Y axis points across the field from the Red Wall towards the Blue Alliance.
+   The X axis points away from the audience to the rear of the field.
+
+Coordinate Position Example
+---------------------------
+
+Let's consider the coordinates (0, -24, 26) in inches on the Into The Deep field, which is a square field.
+Given the order of coordinates then X = 0, Y = -24, and Z = 26. 
+
+The X axis value of 0 is located at the origin in the center of the field.
+The Y axis value of negative 24 would be located closer to the Red Wall, away from the origin by the width of one tile.
+This the center of the wall of the submersible structure on the red side of the field.
+
+The Z axis value of 26 indicates the coordinates refer to the center and top of the Red Alliance "high chamber"
+(which is the higher of the two red crossbars).
 
 Measured Values
 ---------------
 
-The following values have been measured from a 2016 competition field. They are
+The following metric values have been measured from a 2016 competition field. They are
 representative only, and should not be assumed to be exact, or guaranteed.
 
--  Distance between opposite inside faces of panels: 3580 mm
-   (if field assembled well: the straps give some adjustment tolerance)
+-  Distance between opposite inside faces of panels: 3580 mm,
+   (if the field is assembled well: the straps give some adjustment tolerance)
 -  Polycarbonate transparencies have a visible opening height of 255 mm
 -  The top edge of transparencies is 30 mm from the top of the perimeter
 -  Total perimeter height is 313 mm
--  Tiles are 13mm thick
+-  Tiles are 13 mm thick
 
 So, for a diamond field configuration, the corner of the field closest to the
 audience, at a height equal to the top of the perimeter wall, would have a
-coordinate position of: (-1790, 1790, 300).
+coordinate position of: (-1790, 1790, 300) in millimeters.
+
+Additional Information
+----------------------
+
+See this Wikipedia article on `Cartesian coordinate system
+<https://en.wikipedia.org/wiki/Cartesian_coordinate_system#Three_dimensions>`__
+in three dimensions.
+The Field Coordinate System rotation convention comes from the 
+`right hand rule <https://en.wikipedia.org/wiki/Right-hand_rule>`__ 
+of classic geometry.
+
+Robots with a webcam can use :ref:`AprilTags <apriltag/vision_portal/apriltag_intro/apriltag-intro:apriltag introduction>`
+to determine where an :ref:`AprilTag is located 
+<apriltag/understanding_apriltag_detection_values/understanding-apriltag-detection-values:introduction>` 
+with respect to the robot.
+Since AprilTags are in known locations on the field, you can also determine the
+:ref:`location of the robot <apriltag/vision_portal/apriltag_localization/apriltag-localization:apriltag localization>`
+on the field.
+
+Robots can use an inertial measurement unit (IMU) to measure rotations about axes
+with respect to the robot. See :ref:`IMU axes definition. <programming_resources/imu/imu:axes definition>`
+The yaw value from the IMU, also known the heading, measures rotation about the Z axis
+which points up from the robot. 
+You can use the IMU to determine which direction a robot is facing.