Schemas
Much of the complexity in handling LIDAR comes from the need to deal with multiple variables per point. The variables captured by LIDAR sensors varies by sensor and capture process. Some data sets might contain only X/Y/Z values. Others will contain dozens of variables: X, Y, Z; intensity and return number; red, green, and blue values; return times; and many more. There is no consistency in how variables are stored: intensity might be stored in a 4-byte integer, or in a single byte; X/Y/Z might be doubles, or they might be scaled 4-byte integers.
PostgreSQL Pointcloud deals with all this variability by using a “schema document” to describe the contents of any particular LIDAR point. Each point contains a number of dimensions, and each dimension can be of any data type, with scaling and/or offsets applied to move between the actual value and the value stored in the database. The schema document format used by PostgreSQL Pointcloud is the same one used by the PDAL library.
Here is a simple 4-dimensional schema document you can insert into
pointcloud_formats
to work with the examples below:
INSERT INTO pointcloud_formats (pcid, srid, schema) VALUES (1, 4326,
'<?xml version="1.0" encoding="UTF-8"?>
<pc:PointCloudSchema xmlns:pc="http://pointcloud.org/schemas/PC/1.1"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<pc:dimension>
<pc:position>1</pc:position>
<pc:size>4</pc:size>
<pc:description>X coordinate as a long integer. You must use the
scale and offset information of the header to
determine the double value.</pc:description>
<pc:name>X</pc:name>
<pc:interpretation>int32_t</pc:interpretation>
<pc:scale>0.01</pc:scale>
</pc:dimension>
<pc:dimension>
<pc:position>2</pc:position>
<pc:size>4</pc:size>
<pc:description>Y coordinate as a long integer. You must use the
scale and offset information of the header to
determine the double value.</pc:description>
<pc:name>Y</pc:name>
<pc:interpretation>int32_t</pc:interpretation>
<pc:scale>0.01</pc:scale>
</pc:dimension>
<pc:dimension>
<pc:position>3</pc:position>
<pc:size>4</pc:size>
<pc:description>Z coordinate as a long integer. You must use the
scale and offset information of the header to
determine the double value.</pc:description>
<pc:name>Z</pc:name>
<pc:interpretation>int32_t</pc:interpretation>
<pc:scale>0.01</pc:scale>
</pc:dimension>
<pc:dimension>
<pc:position>4</pc:position>
<pc:size>2</pc:size>
<pc:description>The intensity value is the integer representation
of the pulse return magnitude. This value is optional
and system specific. However, it should always be
included if available.</pc:description>
<pc:name>Intensity</pc:name>
<pc:interpretation>uint16_t</pc:interpretation>
<pc:scale>1</pc:scale>
</pc:dimension>
<pc:metadata>
<Metadata name="compression">dimensional</Metadata>
</pc:metadata>
</pc:PointCloudSchema>');
Schema documents are stored in the pointcloud_formats
table, along with a
pcid
or “pointcloud identifier”. Rather than store the whole schema
information with each database object, each object just has a pcid
, which
serves as a key to find the schema in pointcloud_formats
. This is similar
to the way the srid
is resolved for spatial reference system support in
PostGIS.
The central role of the schema document in interpreting the contents of a point
cloud object means that care must be taken to ensure that the right pcid
reference is being used in objects, and that it references a valid schema
document in the pointcloud_formats
table.