Topography Total Station

[Helen Francke]

Thetotal station is the essential tool of a land surveyor. So essential, that it is often referred to as a "gun" by our land surveying cousins over the pond. It typically comprises of an electronic transit theodolite and an electronic distance meter (EDM).

The theodolite measures horizontal and vertical angles, relative to the set up angle. The EDM uses a "time of flight" laser to measure distances to targets. Resulting coordinates are typically saved digitally.

Most total stations come with pre-installed software/apps and options to install more. Combined with the core features - a wide variety of surveying tasks are possible in the right hands. Conventional land surveying techniques are faster and more accurate than ever before.

Before total stations, surveyors needed assistants to help to operate manual instruments. They used theodolites, transits and levels. Distances were measured with metal chains or tapes and software was a notepad and pencil.

Specifications of a modern total station vary according to manufacturer and model. Survey grade models come with the ability to lock to and track a target (glass prism). This allows the surveyor to operate independently where safe to do so.

The main limitations of using a total station to conduct a topographic survey are line of site and range. It cannot measure what the laser cannot reach. Total stations are frequently paired with GPS methods. The surveyor can roam freely and take measurements with the GPS unit which tracks satellites to determine current coordinates.

The data from the total station, or it's remote controller, is output as lines and points. These are usually combined with data from other tools used on site and processed to produce 2d CAD line drawings and 3d Digital Terrain Models. Only the points and lines required to meet the survey specification are measured. Most end users are still using these styles of drawing to inform their decision making.

Laser scanners and drones are transforming the way data is collected by surveyors. Up to millions of points are measured in a fraction of the time taken with a total station. A cloud of points is produced from which dense, data rich deliverables can be provided. Alternatively, points and lines can be extracted from the cloud to produce a more traditional drawing.

The processing of the point cloud can be time consuming, especially where standard CAD drawings are the goal. Points are not independently selected to be measured by the surveyor. Instead, a "scatter" method produces millions of points across the area.

The total station is still the tool of choice to control the accuracy of the data produced by Laser Scanners and Drones. It is likely to feature in a survey project, even if not used as the main data collection tool. Essentially, a wide range of tools allows the surveyor to pick those which complement each other for the task in hand. We expect the total station to be an essential part of that toolkit for many years to come.

A total station or total station theodolite is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure both vertical and horizontal angles and the slope distance from the instrument to a particular point, and an on-board computer to collect data and perform triangulation calculations.[1]

Robotic or motorized total stations allow the operator to control the instrument from a distance via remote control. In theory, this eliminates the need for an assistant staff member, as the operator holds the retroreflector and controls the total station from the observed point. In practice, however, an assistant surveyor is often needed when the surveying is being conducted in busy areas such as on a public carriageway or construction site. This is to prevent people from disrupting the total station as they walk past, which would necessitate resetting the tripod and re-establishing a baseline. Additionally, an assistant surveyor discourages opportunistic theft, which is not uncommon due to the value of the instrument. If all else fails, most total stations have serial numbers. The National Society of Professional Surveyors hosts a registry of stolen equipment which can be checked by institutions that service surveying equipment to prevent stolen instruments from circulating.[2] These motorized total stations can also be used in automated setups known as "automated motorized total station".

Most total station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes etched on rotating glass cylinders or discs within the instrument. The best quality total stations are capable of measuring angles within a standard deviation of 0.5 arc-seconds. Inexpensive "construction grade" total stations can generally measure angles within standard deviations of 5 or 10 arc-seconds.

Measurement of distance is accomplished with a modulated infrared carrier signal, generated by a small solid-state emitter within the instrument's optical path, and reflected by a prism reflector or the object under survey. The modulation pattern in the returning signal is read and interpreted by the computer in the total station. The distance is determined by emitting and receiving multiple frequencies, and determining the integer number of wavelengths to the target for each frequency. Most total stations use purpose-built glass prism (surveying) reflectors for the EDM signal. A typical total station can measure distances up to 1,500 meters (4,900 ft) with an accuracy of about 1.5 millimeters (0.059 in) 2 parts per million.[4]

The coordinates of an unknown point relative to a known coordinate can be determined using the total station as long as a direct line of sight can be established between the two points. Angles and distances are measured from the total station to points under survey, and the coordinates (X, Y, and Z; or easting, northing, and elevation) of surveyed points relative to the total station position are calculated using trigonometry and triangulation.

To determine an absolute location, a total station requires line of sight observations and can be set up over a known point or with line of sight to 2 or more points with known location, called free stationing.[5][6]

For this reason, some total stations also have a global navigation satellite system (GNSS) receiver and do not require a direct line of sight to determine coordinates. However, GNSS measurements may require longer occupation periods and offer relatively poor accuracy in the vertical axis.[5]

Some models include internal electronic data storage to record distance, horizontal angle, and vertical angle measured, while other models are equipped to write these measurements to an external data collector, such as a hand-held computer.

When data is downloaded from a total station onto a computer, application software can be used to compute results and generate a map of the surveyed area. The newest generation of total stations can also show the map on the touch-screen of the instrument immediately after measuring the points.

Most large-scale excavation or mapping projects benefit greatly from the proficient use of total stations. They are mainly used by land surveyors and civil engineers, either to record features as in topographic surveying or to set out features (such as roads, houses or boundaries). They are used by police, crime scene investigators, private accident reconstructionists and insurance companies to take measurements of scenes. Total stations are also employed by archaeologists, offering millimeter accuracy difficult to achieve using other tools as well as flexibility in setup location. They prove crucial in recording artifact locations, architectural dimensions, and site topography.[7]

A total station is used to record the absolute location of the tunnel walls, ceilings (backs), and floors, as the drifts of an underground mine are driven. The recorded data are then downloaded into a CAD program and compared to the designed layout of the tunnel.

The survey party installs control stations at regular intervals. These are small steel plugs installed in pairs in holes drilled into walls or the back. For wall stations, two plugs are installed in opposite walls, forming a line perpendicular to the drift. For back stations, two plugs are installed in the back, forming a line parallel to the drift.

They are most often used in the X and Y axes to lay out the locations of penetrations out of the underground utilities into the foundation, between floors of a structure, as well as roofing penetrations.

Because more commercial and industrial construction jobs have become centered around building information modeling (BIM), the coordinates for almost every pipe, conduit, duct and hanger support are available with digital precision.[clarification needed] The application of communicating a virtual model to a tangible construction potentially eliminates labor costs related to moving poorly measured systems, as well as time spent laying out these systems in the midst of a full-blown construction job in progress.[citation needed]

Meteorologists also use total stations to track weather balloons for determining upper-level winds. With the average ascent rate of the weather balloon known or assumed, the change in azimuth and elevation readings provided by the total station as it tracks the weather balloon over time are used to compute the wind speed and direction at different altitudes. Additionally, the total station is used to track ceiling balloons to determine the height of cloud layers. Such upper-level wind data is often used for aviation weather forecasting and rocket launches.

The Northwest Instrument NTS03 2 Second Reflectorless Total Station is a powerful instrument that can measure distances as far as 1,970 feet in reflectorless mode or up to 16,400 feet with a prism. It has 30x magnification and 2-second accuracy - more than enough for most surveying applications. Up to 20,000 points can be saved in its internal memory or in an SD card. Designed for ease of use, the NTS03 has a laser plummet, Bluetooth, a brightly lit dual-side LCD, and automatic detection and correction for atmospheric refraction and earth curvature.

3a8082e126