Measuring distances with light has been used for terrestrial surveys since the fifties. The instruments measured the distance between itself and a reflecting prism. When the Laser Range finder was introduced in military target acquisition systems a requirement to measure against basically any undefined surface with as few laser pulses as possible was introduced. The military requirement has driven the development of Laser Range Finders and hence generated an instrument technology that is well suited for capturing geographical information.

Profiling systems, since 1970's airborne Laser Range Finders have been used to capture terrain profiles. The principle is basically that a geographically positioned aircraft measures the distance from the aircraft to the ground/object and consequently assign a geographical position to the measured point on the ground.

Scanning area/corridor capturing systems, basically creates a three dimensional digital terrain model (DTM) as the sensor is passing; push broom principle. The leading edge technology needed to build such a system has only recently been made available. Technology enhancements in the following areas have made it possible to make scanning systems:

• GPS positioning systems, accurate XYZ position.
  
• Inertial system, high resolution.
  
• Laser Range Finder; single pulse measurements with high Pulse Repetition Frequency.
  
• Scanning and stabilization systems.
  

The advanced laser systems are made to be eye safe and have LRF:ers with a Pulse Repetition Frequency (PRF) up to 8000 Hz and a capability to distinguish multiple returns from each laser sounding. The absolute accuracy for a single point on the face of the earth can be better than 15 cm.

Key applications
Building digital terrain models is the principle application for any type of airborne laser range finder. A key advantage versus alternate methods is that the laser, used as a "probe", makes it possible to penetrate through a tree canopy and hence finding the true ground. Advanced systems that can discriminate up to four returns from each laser pulse, provides a wealth of data that can be used to determine tree height, bushes, etc.

Applications are opened or augmented because the method circumvents previous obstacles. The advantages in this respect are possibly mainly the time and/or cost reduction concurrent with the technologies inherent advantages - reaching to the ground and measuring objects without texture such as tidal flats. Examples in this area are environmental modelling, related to natural disasters such as flooding, erosion and 3D city modelling for propagation models (telecommunication, noise, pollution ..).
Transmission lines can easily be mapped. One single survey flight can capture the data to:

• Map the location of the transmission towers and the right of way.
  
• Data to measure the line sag for each catinary - can be used in an engineering model to determine the transmission capacity of a segment – ampacity.
  
• Identify potential danger trees - arcing between the conductor and a tree top can create forest fires and electricity outages. From the data set can a work order be generated to dispatch a field crew to take down a specific tree at a defined location (GPS co-ordinates).
  

The same data set can be used for mapping, engineering and operational procedures and thus creates a new dimension in the use of spatial data. It is feasible to establish surveillance systems that on a regular basis monitor a transmission line for various parameters; encroachment, line sag, snow jeopardy, etc.