Lecture 8a - Laser and Optical techniques

Flashcards relating to the lecture on Topographic Data Review

Why is GPS important in geophysical surveys?

An essential part of geophysical surveys, collecting positional data for each measurement.

How can remote sensing and topographic data be used in broader support?

To provide essential background imagery, survey context, or help plan surveys.

What is the benefit of combining geophysics with topographic measurements in active areas?

Give us insight into subsurface processes, while topographic or remote sensing measurements quantify surface effects.

What are the three broad categories of processes that cause topographic change, scaled from continental to local?

Tectonics, Crustal Processes, and Surface Processes

What are two anthropogenic influences on topographic change?

Water abstraction leading to broad local scale subsidence and quarrying leading to site based surface modification.

What methods do we use to measure topography and to detect topographic change?

Using laser-based and optical methods.

Give an example of quantitative estimates of the spatial scales and the rates of change involved.

Mantle processes associated with plate tectonics operate over length scales of thousands of kilometers, with relative plate motions being of order millimeters to centimeters per year.

What is a total station and how does it work?

Combines surveying theodolites with a laser distance measurement to measure the time of flight of a laser pulse to calculate distance.

What do individual measurements from a total station provide, and what is the typical accuracy?

A single 3D point position with respect to the location of the instrument, with an accuracy of a few millimeters.

What was the problem with reflectors at the Soufriere Hills Volcano?

The reflectors would become dirty with ash and water droplets from the volcano, making it dangerous to clean them.

What is a terrestrial laser scanner (TLS)?

A very fast and fully automated total station that uses a laser and an automated scanning platform to make hundreds of thousands of point distance measurements per second.

What are point clouds?

The output data from a terrestrial laser scanner.

Why don't TLS surveys achieve the highest accuracies?

They do not tend to achieve the highest accuracies that total stations are capable of.

How is surface displacement determined when using TLS to monitor a rockfall area?

By the difference between topographic results from repeated TLS surveys.

What did the scanner allow the scientists to assesses about Mount Etna?

Assess how much lava had accumulated in the area.

What is a limitation of terrestrial laser scanners?

They require line of sight, and rugged terrain may prevent observation of some areas from any one location.

How do you avoid data gaps when using TLS?

This can be solved by moving instruments to multiple different viewpoints and combining for one survey.

What is it called when laser scanners are mounted on specialized survey aircraft?

ALS or LIDAR.

What software makes photogrammetric processing easier?

Structure from motion

What type of volcanic hazards do these models help identify?

Evaluate rates of dome growth and identify areas of instability.