Field notes 4 MIN READ
Why a printed route has fewer points than your file
A recording holds tens of thousands of coordinates. A sheet of paper can resolve a few hundred. What gets removed, and what must never be.
A four-hour ride recorded at one point per second is around fourteen thousand coordinates. A 50×70 print at 300 dpi is about 5 900 pixels wide. Something has to give, and it is not the paper.
What is actually removable
Most of those points carry no information about the shape.
On a straight road, a point every second produces a hundred coordinates that all sit on the same line. Remove ninety-eight of them and the drawing is identical, because the two ends already describe the segment.
On a hairpin, every point matters. Remove any of them and the corner opens out.
The classic algorithm for telling these apart is Douglas–Peucker: draw a line between the first and last point, find the point furthest from that line, and keep it if it is further away than a tolerance. Recurse into both halves. Points that survive are the ones that carry direction.
Why the tolerance is a physical quantity
The right tolerance is not a number of points. It is a distance on paper.
If two coordinates end up closer together than the print can resolve, the second one is invisible — it is drawing into the same speck of ink. So the tolerance is set from the print size and the resolution, and works backwards to a distance on the ground.
A 30×40 print of the same ride keeps fewer points than a 70×100 of it, and both look correct, because both are drawn to what the paper can hold.
What we never do
We do not smooth. Simplification removes points. Smoothing moves them, and a moved point is a small lie: the corner you actually took, rounded off to look better.
We do not snap to roads. It would remove GPS drift beautifully. It would also replace your ride with the road network's opinion of it.
And the numbers stay honest
Distance on the print comes from the file's own figure, not from measuring the simplified line. That matters more than it sounds: the privacy trim removes a kilometre from each end, and a distance measured after trimming would report a 222 km ride as 220. The shape is trimmed and simplified. The numbers are not.
Questions people actually ask
- Does simplification change the shape?
- Not visibly. The algorithm keeps every point that carries direction and drops the ones that sit on a line between their neighbours. A hairpin survives; a straight kilometre recorded at one point per second does not need a thousand points.
- How many points end up on a print?
- Enough that the gap between them is smaller than the printer can resolve. On a 50×70 sheet that is a few hundred to a thousand, depending on the route.
- Is this the same as smoothing?
- No. Smoothing moves points to make a line prettier. Simplification only removes points that were adding nothing. We do the second and not the first.
- Does it change the distance?
- The printed distance comes from the file's own figure, not from measuring the simplified line, precisely so that this cannot happen.
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