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Older Rulesets => 5/26/2010 Rules => Rules Discussion 5/26/2010 => Topic started by: Riff Reff on October 07, 2010, 09:34:12 am

Title: Judging distances (20')
Post by: Riff Reff on October 07, 2010, 09:34:12 am
[rule]4.1.1.2 Proximity is defined as not more than ten feet (as measured from the hips) in front of or behind the nearest pack skater.[/rule]

In an older post I presented a diagram (http://euroderby.org/images/stories/misc/packturns.jpg) which shows how distances are measured from hip to hip. Following that principle a skater who is on the outside line can be more than 10' from a skater on the inside line. This is not how it is considered towards pack definition. The important words here are "in front of or behind". After discussing this with a WFTDA referee, I came up with this diagram:

(http://euroderby.org/images/stories/misc/distance.jpg)

It breaks down like this, judging distances is not directly from skater to skater. Instead you don't take the depth of the track into consideration. Imagine a situation whereby, on the straight, there was one blocker on the inside and another blocker 20' ahead on the outside. Physically they would be well over 20' from each other if measured direct from skater to skater, however for the purposes of the Engagement Zone, they are only 20' from each other and therefore the second skater is In Play.

Where it gets tricky is on the corners. Despite the fact that the 10' markings convert the corners into wedges which get wider on the outside, we need to think of the block of space extending from a player as a rectangle. One end of the rectangle pivots around the arc pivot point and carries across the entire track. If you print out the diagram and also cut out a piece of paper the same size as the 20' section of the straight you can kind of see how it works. Place one end of the paper cut-out on the inside arc, and swivel it around the track. At one point it will cover the entire blue section that is shown on the corner.

I know this is really tricky to explain, but hopefully the diagram (and fun exercise!) given above should help work it out.

(If you want the diagram as a PDF-file you can PM me.)
Title: Re: Judging distances (20')
Post by: Whistler on October 07, 2010, 03:59:37 pm
Riff,

Great diagram idea, especially for the straightaways.  In fact, it covers it so well in the straightaways I have nothing to add.

However, I have a few thoughts on the turns...

WARNING!  MATH NERDINESS ALERT

So, what is the appreciable way to consider "In front of or behind" the nearest pack skater on the turns?  I personally don't have a best answer to this, but I would suggest that the answer involve some use of Normal lines to the inside (or outside) curve, in much the same way that illegal clockwise blocks on the turns have to be considered against the normal lines of the curve.  (If you roll across a normal line behind your starting point when blocking, it's a Clockwise Block)

To be clear, the Normal line of a curve, at a point on the curve, is the line perpendicular to the tangent line at that point.  To be more clear, the tangent line is the (only) line that crosses the curve at a single point, thereby defining the "direction" of the curve at that point.

(I know this is all a bit convoluted for the non-math folks, so I'll work on a diagram later today.)

Long Story Short, my point is that it seems like the green 10' block in the turn portion of the diagram doesn't hold up, since I would consider someone at the front-outside corner to be behind someone at the front-inside corner, not even with them.
Title: Re: Judging distances (20')
Post by: Riff Reff on October 07, 2010, 04:59:03 pm
Quote
Long Story Short, my point is that it seems like the green 10' block in the turn portion of the diagram doesn't hold up, since I would consider someone at the front-outside corner to be behind someone at the front-inside corner, not even with them

I see what you mean but that is not what this visualisation is for!
This (let's call it the rectangle) method is a way to visualise how you measure the distance from one skater to another. This is in no way a method to judge possible direction of gameplay violations. Two things are key here:

1. Measure the distance individually for every skater. In the diagram the green rectangle is a tool to measure the distance between the pink pivot and the blocker 12' ahead. For any other skater on the track you will have to swivel the rectangle around.

2. What you actually do is measure the distance between players like in fig.1. Look at it 2-dimensional. Do not consider the depth of the track. fig.2 and 3 are just possible scenarios generated from fig.1


(and for the math nerds: this was done in 5 minutes.. I am a tracer not an engineer.. allow some tolerances)
Title: Re: Judging distances (20')
Post by: Whistler on October 07, 2010, 06:01:50 pm
<Backpedal> Your diagram is ultimately fine for the point you were making, Riff.  It just also raises a more crucial question posed by the language of the rules. </Backpedal>

If we are to measure the distance "in front or behind" someone on the straightaways, this is relatively easy, as direction of the track is, essentially, straight.  Just draw a line, perpendicular to the track edge, from each skater to the track edge and measure the distance between the intersections.

However, this creates a tricky situation on the turns, where have to define the following:


For the first detail, I simply offer the system of normals (i.e. lines perpendicular to the track edge) I mentioned earlier, taken from the inside curve of the track.  If there is such a normal line between the two skaters, whoever is in front of that line is in front of the other skater.

However, in the case of the second detail, it gets hard.  Do you draw a normal line to each skater's hips and measure the distance along the inside line?  Measure the distance along an arc midway between the two track edges?  The problem is that each point (or line) of reference creates different distances.

This may all seem like so much navel gazing, but as long as the rules base proximity off of the distance "in front of or behind the nearest pack skater" (Glossary, "Proximity), we have no choice but to determine a way to figure out how far "in front" of someone a skater is in the turns.  I don't think simple "as the crow skates" measurements would be sufficient.
Title: Re: Judging distances (20')
Post by: Rev. Riot on October 09, 2010, 07:48:44 pm
I think he actually got it from a Level 4 ref...

The only thing in the diagram that I might change is your ten foot on the curve. It should be turned more counter clockwise (? does that make sense, is that a reasonable thing to say?), here's what I mean.

You want to find the foremost skaters spot on the inside line. Then imagine a ten foot line, where that ten foot line would terminate when it intersects with the arc, so that the arc is circumscribing the line. That's the short end of your rectangle.

I say only your ten foot, because it looks like your twenty foot is accurate despite this, since neither end buts up against the curve, but both ends need to be moved the same amount, so you'd cover the same portion of the track either way.