My frame of reference

My credentials, below, do not mean that I am right, but they may affect how you interpret my response. I was a prosecutor in a small county, so I worked on nearly every type of crime and I directly worked with every officer in the county. I read thousands of police reports, I asked for supplemental reports on many occasions, and I trained officers about best practices for writing their reports. Second, my drafting and editing training was unusually extensive, such as being an executive editor on my law school's journal. Third, as a non-lawyer, I have designed curricula, training manuals for teachers, and style guides for websites.

Recasting your issue

I believe that if you change the sentence to active voice, then you will more easily find the words you desire. Furthermore, as a former prosecutor, I beg you to write in the active voice. The officers who read your style guide will be influenced by your style: they will unconsciously tend to adopt the voice you use. Passive voice police reports create problems for prosecutors because in the passive voice, the actor is not always clear: the ambiguity either slows the process, causes confusion, or accidentally misinforms the prosecutor. Active voice is exceptionally useful.

Recasting your sentence in the second-person active voice, "[You should] Describe an object's location in relation to a fixed point--preferably the location of the victim."

We need to pause for a few logistical issues. Unless this style guide is only for crimes that involve one dead body that is present at the scene and was obviously not moved, then your example sentence will have major problems. Second, in some jurisdictions, the word "victim" is prejudicial because it presumes that someone was a victim. If you can, I would avoid that word.

Third, unless you know that all officers are required to carry a compass, I emphatically recommend that you do not encourage the use of compass directions. If they do not all have compasses, in a complex case with multiple reports, you could easily have three reports from three different officers, each with a different direction: north, northwest, north-by-northwest. The discrepancies could be a nightmare for the prosecution because the defense attorney(s) can (and should) use the discrepancies as indicators of doubt.

If your officers do have compasses, I very much like the idea of compass references to the same object at a scene. It is an excellent way to reduce ambiguity. Nevertheless, I would not merely use the word "cardinal" because I firmly believe, based on my experience, that many officers will not know what the word means. If you use the word cardinal, every time you use it, I would define it with an example: "...cardinal direction (north, for example, or southeast)..."

"What do I call that manner of locating?"

You asked, "What do I call that manner of locating?" Ordinal is not correct. Order and ordinal are used to describe the sequence of objects: north, west, westsouth, and southwest do not have a specific order. (In English, the convention is southwest, but in many other languages, the convention is westsouth.)

Technically, only the four principle direction of the compass, north, south, east, and west, are the cardinal directions because cardinal comes to us from Latin cardinālis, "pertaining to a hinge, hence applied to that on which something turns or depends, important, principal, chief." Therefore, technically, if you direct the officers to use the "cardinal directions," they should only use those four directions.

Some form of the word orient (see the third sense, definition 2, especially 2b "to acquaint with the existing situation or environment"), is a more appropriate word than cardinal. (Compare with orienteering.) The original definition of orient was "east," so that strengths the case for this word.

As suggested by others, some version of geography is logical and it would likely make sense to your readers. 4a: "a delineation or systematic arrangement of constituent elements : configuration"

Reducing ambiguity

The directions of a compass are arbitrary: in the common vernacular, we use the magnetic north pole to mean "north", but it is not the geographic northernmost point of Earth, which is yet different from a third point: the geomagnetic north pole. Geographic directions are often based on other things, too. An equally valid definition of "north" is "in the direction of the North Star." The words of a compass are inherently ambiguous because they are arbitrary, therefore, I strongly recommend you look for clear, plain, unambiguous language to describe your desire. If you do, I believe you are more likely to achieve your goal.

If you want the officers to use the points of a compass in their reports, then I believe the best word you can possibly use is "compass."

Conclusion

1. Only recommend the use of terms such as north and southwest if all officers are required to carry a compass.
2. Ordinal does not describe the term "north."
3. Technically, cardinal only covers the four main directions.
4. "Geography" does not inherently include terms such as "north", but it may include that system of delineation.
5. "Orient", originally meaning "east" is a good root word from which to build multiple words to describe your desire.
6. The least ambiguous word to direct officers to use words from their compass is the word "compass."

Coda

Please, I beg you, teach the officers to use active voice. Second, explain that even though Latin words sound more formal, they do not add value to a report, and if the officer misuses the term (think of duplicative and duplicitous), then the formal tone is useless: encourage plain English.

Finally, teach them the difference between words that describe objects and words that label objects (or, in legal vernacular, the words are "conclusive"). "I saw the dead body." is a description but "I saw the victim." is a label (it is conclusive). Officers are the sensory organs ("eyes and ears") of the judicial system, lawyers argue that specific facts (dead body) should have specific labels (victim), but juries are the people we entrust with the final decisions of the meaning of the evidence and its legal consequences.

Every time I explained the last part to officers, they were relieved: it removes a tremendous burden from their jobs. Instead of trying to figure out what everything means, all they have to do is record the things they see, hear, smell, touch, and collect. If they do that job well, then the burden of interpreting the meaning of everything is shifted to the lawyers. (Unfortunately, the US legal system does not encourage lawyers to think or behave this way, but that is not relevant: the cops just need to describe their observations, not make conclusions.)

Good luck

My guess is that no matter how well you write this style guide, it will be unlikely you will receive the appropriate praise for your effort. It is more likely that everyone will find something they dislike but no two people will dislike the same things. Be strong. My heart goes out to you.