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Uncertainty changes the location

03

The optional u parameter explicitly carries uncertainty in meters across all given dimensions.

DIAGRAMRead RFC section 3.4.3 ↗
RFC 5870 · § 3.4.3EXAMPLE
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Uncertainty changes the locationFirst an absent uncertainty parameter leaves uncertainty unspecified despite six fractional digits; the schematic point carries a question. Second an explicit u0 marks an intended specific point, different from the absent value. Third a two-dimensional location with u40 is shown as a circle of radius40 meters and a separate three-dimensional example as a schematic sphere with that radius in horizontal and vertical directions. These are the non-normative GML mappings, not a probability distribution or confidence guarantee. The same single uncertainty applies in all supplied dimensions.Uncertainty is explicit · digits do not set itgeo:48.198634,16.371648;u=402D → GML Circle3D → GML Sphereradius 40 mradius 40 m3D example adds ,183 before ;u=40Schematic · no confidence percentage specified
03
Show the uncertainty region.

For 2D, the non-normative GML mapping is a circle of radius 40 m. Adding a third coordinate makes the mapping a sphere with the same 40 m radius, including the vertical dimension. The diagrams are schematic.

Source for this drawing: RFC 5870 · Read RFC section 3.4.3 ↗

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Terms in this drawing
Uncertainty
An explicitly supplied amount in meters describing how uncertain the location is; no confidence percentage is defined here.
GML
Geographic Markup Language, an XML format. RFC 5870’s illustrative mappings can lose an explicit u=0 when a Point is mapped back.
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KEY POINT

Absent uncertainty and zero uncertainty are never the same.

  • No u parameter means uncertainty is unknown or unspecified. u=0 MAY indicate a specific point in space. Neither means that a measuring device has proved perfect accuracy.
  • A single nonnegative u value applies to every dimension present. The number of coordinate digits MUST NOT be interpreted as the uncertainty level.
  • Non-normative GML mappings use Point for absent or zero u, Circle for 2D nonzero u and Sphere for 3D nonzero u. A GML Point maps back with u absent, so an explicit zero can be lost.
DETAILS

The circle/sphere are schematic, not a promise of a confidence percentage or a probability distribution. The 40 m example is public §6.2. The 3D example adds an illustrative 183 m altitude; u applies to height too. Extra decimal places do not turn 40 m into zero.

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