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

12. 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. Zone A Zone B

13. 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. Zone A Zone B Pick Zone B Zone A Zone B Pick Zone A