We often debate how far we swim when we cross the Ostravice at U Žida and come back. Some say eighty metres, others a hundred, and some reckon it is more. Arguing is all very well, but a metre is a metre, so I decided to measure our route.
The result is 48.6 metres to the opposite bank. Geometrically, that makes 97.2 metres there and back. In a real river, with a current and a swimming line that is never perfectly straight, we surely swim at least a hundred.
What exactly does “across the river” mean?
I did not measure the length of the weir, or a line that looks good on a map but nobody actually swims. I placed the starting point in the middle of the gap in the wall separating the pool from the main flow. This is where we swim out of the pool into the river.
From there, the measurement runs straight across the main channel to the waterline at the wall on the opposite bank. The pool is not included in this measurement of the river's width.
It is 48.6 metres to the other bank
The distance between the two points came to 48.63 metres, or 48.6 metres to the opposite bank, rounded.
That is the measurement shown in the article's main image: from the opening in the dividing wall straight to the opposite bank. The image deliberately includes the whole weir and the left bank, so you can see exactly where the measurement starts and ends.
The arithmetic is simple:
- 48.6 metres across.
- Another 48.6 metres back.
- 97.2 metres in total.
Why we actually swim at least a hundred
The figure of 97.2 metres represents two perfectly straight lines. You could only swim that route if there were no current in the Ostravice, you never adjusted your direction, you turned on the spot at the opposite bank and you returned along exactly the same straight line.
Nobody swims like that.
The current carries us, we continually correct our direction, everyone follows a slight curve and the turn at the far bank adds something too. The actual route only needs to be just under three metres longer than the ideal straight lines to bring us up to a hundred.
Anyone who swims across the river and back at U Žida swims at least 100 metres. Probably a little more.
And the pool? Eleven metres
I measured the pool separately because it is not part of the main channel's width. The line starts where the steps from the left bank enter the water and ends on the pool side of the dividing wall.
The result was 11.04 metres, or 11 metres rounded.
So a single crossing of the pool is approximately eleven metres, or twenty-two there and back.
How I measured it
First, I checked the weir's position and the surrounding boundaries in the Frýdek-Místek cadastral GIS. The cadastral map itself does not show every edge of the weir and dividing walls in sufficient detail, so I used detailed aerial imagery for the measurement: Ortofoto ČR from the Czech Office for Surveying, Mapping and Cadastre.
I marked two endpoints on the orthophoto:
- The centre of the opening in the wall through which we swim from the pool into the river.
- The waterline at the wall on the opposite bank.
I converted the coordinates of both points to the Czech S-JTSK coordinate system, which uses metres. The straight-line distance came to 48.63 metres. Using the same method, I separately measured the pool from the point where the steps enter the water to the dividing wall: 11.04 metres.
I then checked the result using the distance tool on the Mapy.com aerial map. Its rounded reading was 48 metres, consistent with the more precise calculation.
This is not a survey taken by someone standing in the water with professional equipment. For a measurement taken from a map, I allow for a possible error of roughly half a metre. But that is more than accurate enough to answer whether our swim across the river and back is eighty, ninety or a hundred metres.
The figures, simply put
- From the opening in the dividing wall to the opposite bank: 48.6 metres.
- Across the river and back along two straight lines: 97.2 metres.
- Across the river and back when actually swimming in the current: at least 100 metres.
- Across the pool, from where the steps enter the water to the wall: 11 metres.
So next time we are by the water debating whether a return crossing counts as a hundred metres, the answer is simple: yes. At least a hundred, surely.
Ota Zemek