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Merge branch 'draft-1.5' into issue-150-clarify-multiple-extrabyte-vlr
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‎source/01_intro.txt

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@@ -34,6 +34,8 @@ The additions of LAS 1.5 include:
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* Added Max/Min GPS Time fields to header block.
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(`I-118 <https://github.com/ASPRSorg/LAS/issues/118>`_)
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* Added Offset GPS Time definition.
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(`I-6 <https://github.com/ASPRSorg/LAS/issues/6>`_)
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* Clarify requirement that the Extra Bytes VLR must be unique.
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(`I-150 <https://github.com/ASPRSorg/LAS/issues/150>`_)
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‎source/02.04_header.sub

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@@ -91,7 +91,9 @@ Public Header Block
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+----------------------------------+-------------------------+-----------------+-----------+--------------+
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| Min GPS Time | double | 383 | 8 bytes | |
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+----------------------------------+-------------------------+-----------------+-----------+--------------+
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| *LAS 1.5 Header Size* | *391 bytes* |
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| Time Offset | uint16_t | 391 | 2 bytes | yes |
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+----------------------------------+-------------------------+-----------------+-----------+--------------+
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| *LAS 1.5 Header Size* | *393 bytes* |
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+------------------------------------------------------------+--------------------------------------------+
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.. note::
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+-------+-----------------------+------------------------------------------+
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| Bits | Field Name | Description |
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+=======+=======================+==========================================+
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| 0 | GPS Time Type | The meaning of GPS Time in the point |
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| | | records. If this bit is not set, the GPS |
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| | | time in the point record fields is GPS |
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| 0 | GPS Time Type | Used in combination with Time Offset |
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| | | Flag to indicate the meaning of |
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| | | :ref:`GPS Time <gpstime_link>` in the |
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| | | point records. |
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| | | |
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| | | If neither bit is set, the GPS |
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| | | Time in the point record fields is GPS |
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| | | Week Time (the same as versions 1.0 |
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| | | through 1.2 of LAS). Otherwise, if this |
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| | | through 1.2 of LAS). |
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| | | |
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| | | Otherwise, if this |
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| | | bit is set, the GPS Time is standard GPS |
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| | | Time (satellite GPS Time) minus |
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| | | 1 x :math:`10^9` (Adjusted Standard GPS |
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| | | compatibility (which means the CRS must |
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| | | be GeoTIFF). |
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+-------+-----------------------+------------------------------------------+
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| 5:15 | Reserved | Must be set to zero (0). |
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| 5 | Reserved | Must be set to zero (0). |
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+-------+-----------------------+------------------------------------------+
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| 6 | Time Offset Flag | Used in combination with GPS Time Type |
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| | | to indicate the meaning of |
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| | | :ref:`GPS Time <gpstime_link>` in the |
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| | | point records. |
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| | | |
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| | | If both bits are set, the GPS Time |
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| | | is defined as Offset GPS Time. The |
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| | | Offset GPS Time is standard GPS Time |
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| | | (satellite GPS Time) minus |
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| | | 1 x :math:`10^6` x Time Offset. The Time |
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| | | :ref:`Time Offset <timeoffset_link>` |
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| | | is specified in the LAS header. |
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| | | The offset moves time to near zero to |
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| | | improve floating-point resolution for a |
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| | | given time period. |
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| | | The origin of standard GPS |
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| | | Time is defined as midnight of the |
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| | | morning of January 6, 1980. |
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| | | The GPS Time Type flag must be set if |
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| | | the Time Offset Flag is set. |
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+-------+-----------------------+------------------------------------------+
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| 7:15 | Reserved | Must be set to zero (0). |
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+-------+-----------------------+------------------------------------------+
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@@ -264,7 +295,7 @@ The year, expressed as a four digit number, in which the file was created.
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**Header Size**
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The size, in bytes, of the Public Header Block itself. For LAS 1.5 this size is
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391 bytes. In the event that the header is extended by a new revision of the
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393 bytes. In the event that the header is extended by a new revision of the
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LAS specification through the addition of data at the end of the header, the
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Header Size field will be updated with the new header size. The Public Header
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Block may not be extended by users.
@@ -406,3 +437,23 @@ These values must be set to zero if there are no point records in the file,
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if the file utilizes a point record format not supporting GPS Time, or if
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all point records within the file have GPS Time values set to zero,
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such as for certain :ref:`Aggregate Model Systems <aggregate_link>`.
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.. _timeoffset_link:
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**Time Offset**
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The Time Offset field can be used to optimize :ref:`GPS Time <gpstime_link>` precision for a desired time period.
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Offset GPS Time for a point record is equal to standard GPS Time, minus :math:`10^6` * Time Offset.
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For example, Adjusted Standard GPS Time uses a total offset of 1 billion (:math:`10^9`) seconds, which would be specified by a Time Offset of 1000.
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This achieves 100 nanosecond precision for the years 2007-2015. The LAS wiki provides
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`recommended values <https://github.com/ASPRSorg/LAS/wiki/Suggested-LAS-1.5-Time-Offset-Values>`_
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and year ranges that users are encouraged to use to maximize compatibility between datasets.
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This value must be set to zero if there are no point records in the file,
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if the file is derived from a source not utilizing GPS Time
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(such as for certain :ref:`Aggregate Model Systems <aggregate_link>`),
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if the file utilizes a point record format not supporting GPS Time, or if
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the Time Offset Flag is unset.
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It is invalid for the GPS Time Type bit to be unset if the Offset Time Flag bit is set.

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