Glossary

Why a glossary?

This glossary aims to explain commonly used mapping, positioning and drone terms to make them easily understandable.

Accuracy & Precision

These are terms that get bandied around a lot, often without clear understanding of what they mean. They refer to observations such as a geographic location represented by some geographic coordinates.

• Accuracy measures how close a given set of observations are to their true value
• Precision measures how close the observations are to each other

So in the case of geographic coordinates, if I take a series of observations at a known point, I want to know how closely the position I get is to the known position (Accuracy). I also want to see how close each of the observations are to each other (Precision)

Base & Rover

A base is a GPS unit located at a known position and able to broadcast corrections to a Rover which is a moving GPS unit which is able to receive those corrections. Rovers can include another GPS unit or a drone. A base can be a GPS set up over a known location or a nearby CORS.

CORS

A Continuously Operating Reference Station (CORS) is a GPS receiver that is placed on a known location and which broadcasts corrections for the GPS system. In essence a CORS system sits on the known position and receives continous updates from the GPS satellites telling it where it is. What the GPS satellites tell it about where it is differs from where it know it is. This differences is caused by a series of known and unknown errors in the GPS system, such as timing errors, atmoshperic errors and ephemeris errors.

A CORS, because it knows its “real” position, is able to derive a series of corrections to the positions broadcast by the GPS satellites. These corrections can be broadcast to other GPS units to allow them to correct their own positions.

Australia has a network of CORS called AusCORS. Users can connect to AusCORS stations via NTRIP to obtain corrections.

CRS

A Coordinate Reference System (CRS) is a framework used to precisely measure locations on the surface of Earth as coordinates. These coordinates might be latitude, longitude and elevation or northing, easting and elevation.

CRS can be Geocentric (such as WGS84 the system used by GPS) which means they are referenced to the centre of the earth. They can also be “plate fixed” - that is they are fixed to positions on a geological plate. Positions don’t move with respect to the plate, however positions move together with the plate. GDA2020 is the current Australian plate fixed reference system.

When a coordinate system is applied to a map it needs a projection (such as transverse Mercator). The projection determines how points from (essentially) a sphere will be represented on a flat surface. MGA2020 is the current projection system for Australia.

GPS

GPS (Global Positioning System) was originally used to describe the US Military satellite navigation system. It is now generically used as a descriptor for any of the systems of satellite navigation. There are currently several constellations of satellites in use, including, Beidou, GLONASS, GPS, Galileo and QZSS.

GPS is only accurate to 1-2 metres.

GSD

Ground Sampling Distance (GSD) is a measure used in photogrammetry. In the simplest terms it is a number which tells us - in centimetres or some other unit of measurement - what distance on the ground is represented by each pixel on the camera sensor. The pixel is the smallest unit of light acceptance on the camera sensor, and thus the smallest level of resolution.

A GSD of 1cm will show much more detail than a GSD of 2.5cm. The smaller the measure, the higher the effective resolution. In addition there is a rough rule of thumb that your position accuracy cannot be better than 2 times your GSD.

It would seem then that every job should strive for the lowest GSD. However delivering a low GSD requires flying lower to the ground, which means each photography run must be closer to the last run, in order to maintain coverage.

Halving the GSD can easily require double or more flight time, double or more processing time and therefore at least doube the total job cost.

It’s important to determine what level of GSD is actually required for the intended use of the photogrammetry. That way the job only costs as much as it needs to and no more.

Known Point

A known point is a geographic location where the position has been accurately observed and the position marked on the ground. Government survey marks are one example of a known point.

It may be necessary to establish a known point before commencing a particular job if pre-existing known points are not available in close proximity.

NTRIP

NTRIP is a network protocol to broadcast GPS corrections over a network - such as the internet.

Orthomosaic

An orthomosaic or orthophoto is a series of aerial images that are geometrically corrected and stitched together to create a single image. Because they have been geometrically corrected they have an accurate scale and objects in the image correctly relate to each other.

Orthomosaics are created with reference to a particular coordinate reference system (CRS).

Photogrammetry

Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images. In our case, generally aerial images.

RTK

RTK stands for Real Time Kinematic. RTK is a way of correcting a GPS signal so that the accuracy of the location obtained improves for 1-2m with GPS to 3-5cm of theoretical accuracy with RTK. RTK does that by taking correction signals from a reference station at a known point represented by a CORS and feeding them to a GPS unit. The GPS unit in turn corrects its position on the basis of the corrections it receives.