In short, if the coordinates (in the desired datum) of any two or more physical points are known, then the datum can be reestablished using the computational abilities of the data collection software.
There are two principal means of accomplishing this task. The first, and easiest, is to set up the instrument on (occupy) one known point, and then backsight to another. Using the known coordinates of the points, the data collector inverses between them to determine the distance and direction from the occupied (reference) point to the backsight (reference measurement) point. With the known location (coordinates) of the occupied point and direction to the backsight point, the original datum is re-established upon completion of the backsight reading (reference measurement).
The second, more esoteric, procedure is known as “free-stationing.” In this situation, the instrument is placed in any suitable location from which two or more known points can be observed. Through a mathematical procedure (resection), the data collector calculates both the location and orientation of the instrument by means of the observations to the known points. We will examine the first option in this installment, and save the second for later.
Here is the crash scene that we will use for this example:
(Click on images for larger version)
We will assume this is an intersection that sees a lot of crashes (and, therefore, gets mapped a lot!). On the first visit to this scene, we set up reference and reference measurement points:
From the last article, we know that this initial setup creates a datum (coordinate system) like this:
Where the origin (0,0,0) is at the reference point, and the +Y (Northing) axis is in the direction of the reference measurement [note: this statement assumes a reference direction of 0, or North, in the setup, but the following procedures are effective regardless].
The scene is then mapped from this reference point, creating points at the locations of the mapped items:
The linework of the scene is then generated based on those points:
Now, at some later time, a re-visit to this scene is necessary. It could be to map additional points for a line-of-sight study, or to pick up additional points for a 3D scene. Or, maybe the scene was originally mapped in order to create a template drawing of the intersection. Either way, we need to set up on this scene again and measure additional points, and we want those points to be in correct relation to the points in the original file.
To do this, we first want to re-establish the datum from the original mapping job. I think everyone would agree that the easiest way to do this would be to set up over the original reference point and backsight to the original reference measurement, right? But, what if the marks are no longer there? Or maybe a crashed vehicle is sitting on top of one of them?
As stated above, all that is needed to re-establish the datum is two points whose coordinates are known to the data collector. By re-opening a copy of the original scene in EvR, all of the previously-mapped points are available. We must then select two previously-mapped points that we can locate to a fairly high degree of accuracy. In this example, we will use the ends of two roadway lines:
As we now know, given any two points having known coordinates in the original datum, the distance and direction between them can be calculated. The direction, or azimuth, is simply the horizontal clockwise angle between “north” (i.e., the Y axis) and the target point:
This is really nice to know, but the fact is that EvR does all this for us. All we have to do is tell the data collector what points we are using, by entering point numbers. In this case, we will set up on point 100 and backsight to point 197. In EvR, in the Occupy Reference Point menu:
Here, we entered point number 100 in the Ref Pnt (Occ) field. Because there are coordinates present in the file for point 100, EvR tells us so with the notation “XYZ Defined” to the right of the field.
We then enter point number 197 in the Ref Meas (BS) field:
and we again see that there are coordinates present for this point, based on the “XYZ Defined” notation.
Note that EvR automatically calculates and displays the direction from the RP to the RM, based on the coordinates of the point numbers entered:
As we discovered earlier, this is the key to re-establishing our datum!
At this point, all that is needed to complete the setup data is the instrument height. However, we will take a look at some other information that EvR gives you. By picking the “Bs” (backsight) tab, the point number, description, and coordinates of the chosen reference measurement point can be examined:
Likewise, picking on the “Occ” tab will yield the same details for the reference point:
Now, we can pick on “OK” and go on to complete the reference measurement:
Again, EvR shows the IDs of the reference and reference measurement points, as well as the reference direction.
Following normal FM procedures, we sight the reference measurement target and tap “Shoot Angles and Distance.” EvR then displays the residuals, or the differences between the expected (calculated) RM distance and elevation, and those that the instrument actually measured. This screen is the same as that seen when completing a move from one RP to another. We must examine the residuals to ensure that they are within an acceptable range. This is also a good means of ensuring that no blunders have been made along the way (i.e., wrong point numbers entered, etc.). Basically, if the residuals are quite small (small fractions of a foot, as we have here) then the setup is OK.
We then can proceed to map the evidence from our crash from this setup, using normal procedures:
And when this file is transferred to MapScenes, all of the points collected will be in their proper locations with respect to the others!
One area of concern when using these (and other) procedures to re-establish a datum is the accuracy to which we can locate the points used for the ‘new' reference point and backsight. It should go without saying that we must be as precise as possible when setting up on these points, in order to obtain sufficient accuracy.
In the case of scene templates, a good practice for ensuring this level of accuracy is to establish control points during the original (scene) mapping job. In this context, control points are points that are established and mapped solely for the purpose of re-establishing the datum during subsequent visits to the scene, and are monumented, or marked, in some fashion that will allow them to be recovered accurately days, weeks, or even months later. Masonry nails (P-K nails or MagNails) can be used for this purpose on asphalt pavement, and short lengths of iron pipe or rebar can be driven into softer surfaces adjacent to the roadway. If used, control points should be established in several locations about the scene in accessible locations, with as much separation as possible, and each having line-of-sight to one or more of the others:
In the next installment, we will examine the use of these same procedures to map a large-scale scene using multiple instruments.