Every geomantic figure is four rows of one or two points, so each of the sixteen can be described exactly, without reference to any interpretation, by listing its rows.
The pattern
Cauda Draconis is single, single, single, double, reading Fire, Air, Water and Earth from the top down. That is five points in total. The three light rows above one heavy row give the figure a narrow head and a broad base, the mirror of Caput Draconis, and the shape is where both names come from.
Flipping every row
Change each row from single to double and from double to single and the figure becomes Tristitia. Each of the sixteen has exactly one partner under this operation, so the set falls into eight pairs, and this is one of them. Authors use different Latin and English terms for the operation, which is why this atlas describes what is done to the points rather than adopting one writer's vocabulary.
Why the pairing is structural and not interpretive
Nothing in the traditional readings of Cauda Draconis and Tristitia requires them to be read as a pair. The relationship is arithmetic: four rows with two states each give sixteen figures, and inverting the state of every row is a one-to-one mapping of the set onto itself with no fixed points. Some modern handbooks make use of the pairing in reading a chart; the older Latin sources largely do not.
The atlas records the element, planet and sign for this figure separately. Those attributions were laid over the figures by the astrological tradition and the sources disagree in places, whereas the point pattern above is intrinsic and can be checked by counting.