You need to triangulate all evidence and clues found in relation to the scene. When triangulating, you need to use for each piece of evidence being recorded two permanent objects (reference points), like a large boulder, tree, or utility pole, that are not likely to be moved. These reference points should not be too far from the evidence (no farther than 100 feet if possible). The two reference points and the piece of evidence form a triangle, hence the term triangulation. If you don't have to permanent objects, you can make you own reference points by using a two-foot piece of rebar and pound it into the ground as a reference.
Whatever object you are measuring to or from, use the same spot on the object every time.
Many investigators carry spray paint, roofing nails or other marking devices with them to mark the location of their reference point in case they have to return later. If you can, use the same two references for all your evidence, but many times this is not possible. Make sure you note on your crime scene sketch where your references are located and provide a description of each.
Use your measuring tape, which should be nonmetallic, along with your compass. I prefer to use a compass board with an extended lubber line (lifeline). By placing the tape at your reference point, extend the end of the tape to your evidence placing the end over the center of the evidence. Then lay your compass directly over the measuring tape and record the compass bearing from your reference point to the evidence and record the distance. Once this is done, move to your second reference point and repeat the procedure.
Evidence in water is marked by a marking buoy. You measure from the reference point to the buoy floating on the surface. You do not take the tape underwater to the evidence. If the water is shallow, however, you can stand over the evidence. In the figure below, the base of the tree is the reference point and the diver in the water is standing over the evidence.
Now measure the base of the triangle, which is the distance from reference point to reference point. In this way, you are triangulating the item's location and if done correctly, this will allow you to return later to that exact location.
When you are triangulating from a bridge, it might not be feasible to use a measuring tape. A diver on the bridge would simply shoot an azimuth to the marker buoy for the evidence using a known point on the bridge, like one of its pillars. Shoot another azimuth using another point on the bridge and you can then effectively triangulate the location of the evidence.
A diver in the water, while he is floating over the evidence, or someone on a boat can do the same thing by shooting azimuths back to the bridge.
Triangulation is very easy once you have done it a few times. Make sure all your team members know how to triangulate so they can triangulate their evidence and put this information on their crime scene sketch.