Python Tutorial

This basic tutorial explains how to use libLAS to read and write LIDAR data encoded in LAS file format from Python.

Reading

1. Reading LAS data in Python is a simple as opening the file with the file.File class and using the iterator to chug through the points.

>>> from liblas import file
>>> f = file.File('file.las',mode='r')
>>> for p in f:
...     print 'X,Y,Z: ', p.x, p.y, p.z

  1. You can also read specific points from a file:

>>> from liblas import file
>>> f = file.File('file.las', mode='r')
>>> p = f.read(0)
>>> p
<liblas.point.Point object at 0x7377f0>

File versions and formats

The LAS format, as of this writing, provides three different file format versions (1.0, 1.1, and 1.2) and four different point format verions (0, 1, 2, 3). It is important that you be mindful of the minor_version and the dataformat_id when working with files. If the dataformat_id is not correct for the type of data you want to store (color, time, or color + time), none of that data will be read or written, even though placeholders will exist on the liblas.point for them.

Point format versions and their properties

dataformat_id

Time

Color

0

1

x

2

x

3

x

x

You control whether or not color, time, or color + time is stored in the file by setting the dataformat_id in the header that you use to create the file.File. This has to be done when you instantiate the file, it cannot be changed after the fact. Attempting to do so may result in disastrous results.

Base properties of all points regardless of dataformat_id

Name

x

y

z

intensity

return_number

number_of_returns

scan_direction

flightline_edge

classification

scan_angle

user_data

Point

The liblas.point module contains a Point class that you can use to manipulate LAS point data. It is fairly basic and contains a number of properties you can set and get:

>>> p.x, p.y, p.z
(289814.15000000002, 4320978.6100000003, 170.75999999999999)

>>> p.scan_angle
0
>>> p.scan_direction
0
>>> p.return_number
0
>>> p.number_of_returns
6
>>> p.flightline_edge
0
>>> p.classification
2
>>> p.time
datetime.datetime(1970, 1, 6, 12, 44, 10, 1)

>>> p.intensity
120

>>> c = p.color
>>> c.red
255
>>> c.blue
255
>>> c.green
255

VLRs

Variable Length Records (VLR) are frequently used by applications to store anything they wish in the file as a “blob” written into the header of the file. libLAS supports writing and creating your own VLRs in addition to taking on the work of interpreting and using VLR records related to spatial reference systems if GDAL and proj.4 are linked into the library.

The following code demonstrates how to write your own VLR by opening an XML file and inserting it into a new file.

from liblas import file as lasfile
from liblas import vlr
from liblas import header as lasheader

f = lasfile.File('test/data/srs_utm17.las',None,'rb')
h = f.header

v = vlr.VLR()

text =  open('schemas/las.xml','rb').read()

import ctypes

data = ctypes.create_string_buffer(text)

v.userid='hobu'
v.recordid = 12345
v.data = data

h.add_vlr(v)

f2 = lasfile.File('junk.las',header=h,mode='w')
for p in f:
    f2.write(p)
f2.close()

Writing

To write a new LAS file, you are first required to have a header. The header will have a number of default values, but it is important to set the dataformat_id and version_minor if you wish to have 1.1 files or records that also include time values.

>>> from liblas import header
>>> h = header.Header()

### Support storing time values
>>> h.dataformat_id = 1

### Store a 1.1 version file
>>> h.minor_version = 1

Another important item to not is possible to have the same file open for read and write at the same time because LAS files are sequential. For example, the following will fail:

>>> f = file.File('junk.las', mode="w", header=h)
>>> f2 = file.File('junk.las')
Traceback (most recent call last):
...
LASException: ('File %s is already open.  Close the file or delete the reference to it', 'junk.las')

Writing to a LAS file is as simple as opening the file for write mode with the header you want to write and issuing the write() command with some liblas.point.Point instances:

>>> f = file.File('junk.las',mode='w', header= h)
>>> pt = liblas.point.Point()
>>> f.write(pt)
>>> f.close()