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PIDF-LO Profile

PIDF-LO Profile. James Winterbottom, Martin Thomson IETF-65. What’s Changed?. Updated the rules based on comments to make them less ambiguous. Removed much of the geospatial section and made extensive references to the GeoShape draft.

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PIDF-LO Profile

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  1. PIDF-LO Profile James Winterbottom, Martin Thomson IETF-65

  2. What’s Changed? • Updated the rules based on comments to make them less ambiguous. • Removed much of the geospatial section and made extensive references to the GeoShape draft. • Modified examples to be in line with Revised Civic draft and GeoShape draft.

  3. Issues • Recommend a restriction of polygon to 16 points when transporting own location. • Do we want to recommend shape types for emergency calling?

  4. LCI and Uncertainty

  5. LCI Uncertainty • In LCI (RFC 3825) defines a resolution that describes a region of uncertainty. • Appendix A of PIDF-LO Profile describes how to get a PIDF-LO from LCI. • Resolution indicates a number of bits that can be considered valid.

  6. Converting from LCI • (From RFC 3825) 38.89868° is • 000100110.1110011000001111111001000 • A resolution of 18 indicates a range using the first 18 bits: • 000100110.111001100xxxxxxxxxxxxxxxx • x bits can have any value from all 0s to all 1s: • 000100110.1110011000000000000000000 to 000100110.1110011001111111111111111 • 38.8984375° to 38.9003905951976776123046875°

  7. Converting to LCI • (RFC 3825 doesn’t include any example of this) • When converting a value with uncertainty to LCI, start with the maximum and minimum values: • 32.98004° to 32.98054397° (~56 metre range) • 000100000.1111101011100011111001110 to 000100000.1111101100000100111011100 • Resolution is the number of identical bits: • 000100000.1111101, which is 16 bits • Converting back to check accuracy: • 32.9765625° to 32.9843745° (~870 metre range)

  8. That wasn’t a contrived example • That was a randomly selected point. • This gets much worse for certain border cases: • 31.9999985° to 32.00000274° (0.5 metres) • 000011111.1111111111111111111001110 to 000100000.0000000000000000001011100 • The resulting resolution is only 3 bits! • Conversion gives a range from 0° to 31.9999999701976776123046875°… • The error has increased to 3,500,000 metres!

  9. Conclusion • LCI encoding has a major flaw. • This flaw causes it to break down near certain boundaries. • This problem can occur all over the world wherever the minimum and maximum values for a location differ in too many bits. • It doesn’t matter what the original precision was! • A location with uncertainty that spans the Greenwich Meridian or the Equator cannot be represented: • Resolution = 0 bits! • Resolution is effectively useless.

  10. For my next trick • I will demonstrate how important uncertainty is. • …and why it should not be ignored.

  11. Zone B Zone A Where is the Target: Zone A or B? • This is easy: it’s clearly in Zone A, right?

  12. Zone B Zone A Where is the Target: Zone A or B? • If you include uncertainty, the answer is less clear. • To be certain, the overlap with Zone A must be compared with the overlap with Zone B.

  13. Pick Zone B Pick Zone A Zone B Zone B Zone A Zone A Where is the Target: Zone A or B? • The amount of uncertainty determines which zone the Target is most likely to be in. • The bigger the uncertainty, the more likely the Target is in Zone B.

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