The last pattern in Lab 9 looks like this with all the centers marked.
In the original square ABCD, the lined BC and CD are mirror lines for line reflections, and A is a 90-degree center of rotation. The points B and D are not centers of rotation.
The invariant lines for glide reflections are
This pattern is extended throughout the plane by rotating by 90 degrees with center A and reflecting in the mirror BC to get all the thick red line mirrors and thin line invariant lines.
The centers of rotational symmetry are the halfturns centered at the intersection of the thick red mirror lines (i.e., the centers of the rhombi) and the 90-degree rotational centers at the centers of the "red squares" of side 2 formed by the mirror lines.
The translational symmetries are the translations that take blue 90-degree centers to other blue centers. These are combinations of the horizontal and vertical vectors of length 4, such as 4DC and 4AD. The horizontal and vertical vectors are compositions of line reflections in parallel mirror lines.
The hardest glide reflections to visualize are the ones along diagonals of the original small squares, such as line BD or line B'D' in the figure below.
In this figure two right triangles in the rhombi have been colored; the image of triangle C'D'M is triangle C'''B''M'' by the glide reflection GB'D'. Also, a triangle AD'M' has been added to show an intermediate stage in producing the glide reflection. This triangle is the reflection of C'D'M across line B'D'. Then this triangle is translated by vector B'D' (which = vector D'B'') to produce triangle C'''D'''M'' by the definition of the glide reflection GB'D'.
One can see then how the whole rhombus containing D'C'M is moved by the glide reflection to the rhombus with long diagonal BB''.
A Java Sketchpad Animation is also available for this glide reflection.