[Liblas-devel] File creation date

[Mateusz Loskot]

Hi,

Could anyone clarify the difference (or no difference) between
Flight Date Julian attribute defined in LAS 1.0
and
File Creation Day of Year defined in LAS 1.1

Size of both attributes is 2 bytes.

My senses tell me that in both versions, this attribute stores
exactly the same value - Day of Year. But, if I'm correct, then the LAS
1.0 Standard is incorrect or imprecise.

The Julian Day (JDN) is number of days since Jan 1, 4713 BC.
The Day of Year, as ordinal date is number of day since the beginning of
a year, usually current year and ranges from 1 to 365 (366 - leap years).

Why I think the "Julian Day" in LAS 1.0 is not real JDN but non-Julian
ordinal Day of Year? Because unsigned integer of 2 bytes is too short to
hold JDN number. 2 bytes can store up to 64535 (as unsigned), but today
JDN is 2454478. this value is too big.

Summarizing, is it correct to assume that in both LAS 1.0 and 1.1,
value of attribute number 12 (Flight Date Julian 1.0 or File Creation
Day of Year in 1.1) can store only values from 1 to 365 (366) ?

BTW, I've noticed that in all LAS 1.0 data I have, none of date
attributes (fields 12 and 13) are set. Only LAS 1.1 data I have does
include value of this attribute. Is this a common practice?

Cheers
-- 
Mateusz Loskot
http://mateusz.loskot.net