Why a 3 Metre Grid Cell Is Practical

A cell of 3 metres by 3 metres is small enough to separate one entrance from the next, yet large enough to match the accuracy that people and devices can really achieve. A cell much larger would blur neighbouring doors together, and one much smaller would claim precision that no phone or map click can back up.

The size problem

Choosing a cell size is a trade-off between three things:

1. Discrimination: can two nearby places have different codes?
2. Reliability: will the same place give the same code when different people select it?
3. Convenience: how long must the code be, and how easy is it to use?

Small enough to distinguish places

On a typical street, the width of a house frontage, a shop door or a gate is a few metres. A cell of about 3 m is comparable to that scale, so adjacent doors or gates often fall in different cells. Compare that to a cell of 10 or 15 m, where several shop fronts may share a code. For delivery, the difference matters: the code points to one gate, not to a stretch of road.

Large enough to be reliable

Consumer GPS is often accurate to within a few metres in open sky, and worse among tall buildings; see why phone GPS is inaccurate. Selecting a point on a zoomed-in map is usually even more reliable, as covered in map zoom levels. A 3 m cell is around the scale of that uncertainty, so it is a fair match to what is achievable. A 30 cm cell would suggest centimetre accuracy; two people choosing the same doorway would rarely land in the same cell, and codes would appear to disagree.

Short enough to share

Every halving of the cell size adds to the information a code has to carry. At a few metres of resolution, a code can still be kept to a short string of letters and digits, which can be typed on a phone, read over a call or printed on a parcel. Coordinates with five or six decimals, compared with that, are long and error prone; see decimal places and precision.

Same cell, same code

Because everything inside a cell shares its code, small differences in where two people tap on the map do not matter, as long as both end up inside the same 3 m by 3 m square. This tolerance is what makes a code useful as a shared reference. The flip side is that a code names an area, not an exact coordinate.

Choosing the right cell

The 3 m cell describes how finely the code divides space, and the code always leads to that exact square. To get the best result, zoom in and pick the gate or entrance itself. UPCS does not replace the written address, which still carries street, number and flat details.

For how cells are numbered and written as codes, see how location codes are generated from coordinates.