Positioning and reference/ Geodesy

Cartesian and three-dimensional geodetic coordinates

The best known terrestrial reference systems are :

- NAD83 (North American Datum 1983);

- WGS84 (World Geodetic Datum 1984);

- ITRS (International Terrestrial Reference System).



The terrestrial reference system is materialized on the territory with the help of landmarks. All these markers form the geodetic network. In Quebec, the Ministry of Natural Resources ensures the integration and dissemination of information related to the geodetic network.

Today, the measurements used to determine the position of a landmark in a terrestrial reference system or to position oneself in it come mainly from these four space techniques :

- VLBI (Very Long Baseline Interferometry): very long baseline radio interferometry;

- SLR (Satellite Laser Ranging): satellite laser telemetry;

- DORIS (Doppler Orbitography by Radio-positioning Integrated on Satellite): positioning system based on the Doppler effect;

- GPS (Global Positioning System), probably the best known (global positioning system).

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The field of gravity

The gravity field plays an important role in the shape of the Earth. The Earth is a complex plastic body composed of solid, semi-rigid, and liquid parts. Since these different parts are not made of identical materials, the gravitational field differs from one place to another.

The measurement of gravity makes it possible to define another surface representing the shape of the Earth: the geoid. This surface gives an approximation of the mean sea level. It is made up of hollows and bumps caused by the uneven distribution of masses of various densities.

Moreover, being a plastic body, the shape of the Earth changes continuously. It is subject to different internal and external forces.


For a person, these changes may be perceptible, such as the tide, or imperceptible, such as the land tide (amplitude of about 30 cm) and the drift of lithospheric plates (tectonics). To track the movement of the plates, a four-dimensional reference frame (three axes and time) independent of the earth's surface is required. The International Terrestrial Reference System (ITRS) has this temporal capability. Unlike the ITRS, the NAD83 system is fixed to the North American tectonic plate. The position of the points is invariable in time unless they are located on an unstable part of the plate.

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The GPS

In the past, the stars allowed navigators to position themselves. Today, GPS satellites have replaced the stars and unlike the stars, satellites are accessible 24 hours a day despite the clouds.

The arrival of the GPS (Global Positioning System) has revolutionized the way of doing things in the field of positioning and navigation. Set up, controlled, and operated by the United States Department of Defense (DoD), the GPS system is based on a constellation of satellites that allow a position to be determined at any time and in any place on Earth. This constellation is made up of at least 24 satellites operating at an altitude of approximately 20,200 km.





Each satellite transmits on two carrier waves different coded messages containing civil and military information, including its position in the WGS84 terrestrial reference system. Civilian information is broadcast on a single carrier wave.

The principle for obtaining a position is very simple. Using the coded message transmitted by a satellite, a GPS receiver determines the time it took for the signal to travel the distance to that satellite, approximately 0.07 seconds. By multiplying the time obtained by the speed of light (300,000 km/s), the receiver can determine this distance. Thus, measurements on four satellites allow the position of the receiver to be determined. The accuracy of this position varies from ten meters to one centimeter and better depending on the type of receiver used.

When calculating its position, the GPS receiver synchronizes its clock with the atomic clocks of the GPS satellites. Once synchronized, the receiver displays the time with a very high degree of accuracy and in turn, becomes a very precise time reference.

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Consumer receiver

The vast majority of users of GPS receivers have a positioning need of the order of ten meters. This is the accuracy obtained with a single receiver reading the coded message from a minimum of four satellites.

Receiver for differential positioning

Another level of accuracy is between 0.5 and 5 meters. To obtain it, it is necessary to read the coded message with two GPS receivers simultaneously. This is called differential positioning.

Receiver for carrier wave positioning

Carrier wave positioning is the most accurate, ranging from 30 cm to better than one centimeter. Instead of reading the coded message with two receivers, the receivers measure the wave that carries the coded message. Usually, these are specialized receivers. However, this method is very sensitive to obstructions that can cut off the signal.

Differential positioning and carrier wave positioning requires the processing of data collected by both receivers. 

This processing can be done in specialized software once all the data has been collected, commonly called post-processing, or in real-time if there is a communication link between the two receivers. 

This link is used to transmit data from one receiver (called the base) to the other. The receiver that receives the data (called the cell phone) has software to process the received data on its own. 

The accuracy of the position can vary from a centimeter to about two meters depending on :

- the distance between the two receivers;

- the time it takes to capture the data;

- the type of receiver used in the field;

- the way the data is processed.

The Ministère des Ressources Naturelles has set up permanent GPS reference stations throughout Québec to serve as a base for applications requiring differential positioning or carrier wave positioning. These stations allow positioning by post-processing and the majority of them offer real-time centimetric positioning by cellular telephony.

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Why use geodesy and GPS?

Although little known to the general public, geodesy is the basis of everything related to terrestrial positioning.

 A reference system resulting from geodesy allows to position oneself and then to position different works in a common reference frame.

 Its infrastructure of points (geodetic network) is indispensable not only for the precision and homogeneity of maps but also :

- to navigation (land, sea, or air);

- geophysics;

- Space science;

- Atmospheric sciences;

- Prospecting;

- knowledge of continental drift;

- monitoring of oceanic phenomena;

- everything related to earth sciences.

Thus, for geophysics, geodesy provides :

- information for the study and modeling of the deformation of the Earth's crust over time;

- Gravimetric data to study irregularities in the distribution of crustal density and its variation over time.

For space sciences, geodesy provides :

- the geometry of the gravitational field necessary for the prediction of satellite orbits. This contribution will further increase in view of the current and future exploitation of increasingly efficient space resources.

The advent of geographic information systems has exploded the need for location in a single repository in order to integrate and share data from different sources.

Thanks to the advent of GPS, the notion of positioning has become democratized.

There is a multitude of fields in which this cutting-edge technology is used with or without the help of a geodetic network :

- air transport

- cartography

- surveying

- navigation

- telecommunications

- remote sensing

- regional/recreotourism development

- civil security

- agriculture

- forestry

Pilots use GPS to guide their aircraft.

Geodetic points are used for photogrammetric purposes, thus for map-making and the collection of data from geographic information systems.

GPS and geodetic points are used, among other things, for topographic surveys, subdivisions, and infrastructure construction (bridge, road, dam, etc.). Cities and other service agencies use them to locate their infrastructures such as power lines, fire hydrants, and even streets.

At sea, fishermen and sailors use them as a guide to navigate or to locate places.

GPS receivers are used to synchronize telecommunications networks.

The addition of GPS receivers aboard low altitude satellites allows for more precise measurement of their location and more accurate analysis of their orbit.

Hikers, campers, and hunters use it to avoid getting lost.

It is used to track vehicles in real-time (fire, ambulance, or other 911 services), to locate persons in distress, etc.

Farmers use it to better manage their farms (precise application of seeds and herbicides, etc.).

Foresters use geodetic points and GPS to carry out forest inventories and to locate forest roads.

satellite images / Remote sensing

Remote sensing

Remote sensing is used for Earth observation. This is a technique that allows you to acquire information on a ground target from a satellite.

Information received by satellites is digitally recorded using electronic sensors.
The information is then recorded as an image and transmitted to receiving stations on the ground. Onboard satellites, the sensor records images in the form of spectral bands.
Each of the spectral bands gives a black and white image, where the shades of grey vary.
 By superimposing three spectral bands with color filters, it is possible to obtain a color image or a color compound.
Each colored compound gives different information on land use.

Lake Lanyan - Nord-du-Québec 

Gaspé (Eastern Quebec)

•  the forest domain: hardwood, mixed, softwood, and forest cuts;
•  the agricultural domain: maize, potatoes, hay, cereals, wasteland, pastures;
•  hydrography and major roads;
•  urban agglomerations;
•  peat bogs.

The resolution of the images

•  the image taken by a satellite at a resolution of 10 meters makes it possible to observe the  location of the land and its environment;
•  the image taken by an aircraft at a resolution of one meter allows the baseball field to be circumscribed and details to be seen within the field itself.

Why use satellite images?

•  agriculture 
•  forestry 
•  geology 

agriculture 

•  classify the main types of culture;
•  assess crop health;
•  estimate the total production of a crop;
•  map soil management practices;
•  monitor compliance with laws and treaties.

forestry

geology

•  mapping of surface deposits;
•  mining and petroleum exploration;
•  environmental geology;
•  mapping and monitoring.

These images from the Megantic and Orford region show a simulation of what can be seen by looking at two three-dimensional aerial photos, without stereoscopes or glasses to see in 3D!

- The images on the left are mosaics of about twenty color aerial photographs at a scale of 1:15,000, which show a large area in one piece.

- The images of the center are called Digital Elevation Models (DEMs) and were produced from the contour lines found on the 1/20,000 scale topographic maps. These MNA present, in images, the relief of these regions.

- On the right, the result of the combination of the MNA and photo mosaics: Mont Mégantic and Mont Orford in perspective and in all their splendor!

Mont Mégantic
Mont Orford
Another way to see aerial photos in 3D and, this time vertically, as you would see them with a stereoscope, is the anaglyph process. This word is taken from the Greek word anaglyphs which means "carved" or "embossed".

In these examples, it is an overlay of two consecutive aerial photographs (the area where there is a longitudinal overlap) of Mount Orford on the left and the Hautes-Gorges-de-la-Rivière-Malbaie on the right. The use of two complementary colors, here red for the left image and cyan (blue-green) for the right image, allows the relief to be reconstructed by looking at the composite image through "3D" glasses equipped with colored filters. Each eye sees only the image that suits it and the brain merges the two images to give the illusion of a single relief image.

Note: Red-cyan anaglyph 3D anaglyph glasses are available in different locations, such as photoshops, or on the Internet. But maybe you had some with your latest DVD? in 3D? You can also make your own glasses using red and cyan acetates, quite simply!

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The different types of aerial photos

Aerial photographs are images of the territory taken on a given date and are used to interpret the forms and details that characterize it. The interpretation of shapes, shades of color or grey and textures makes it possible to distinguish the details present and their nature. For example, it can be interpreted:

•  hydrography (lakes, rivers, streams, ponds, ponds);
•  vegetation (woodland, wetland, crops);
•  inhabited areas;
•  transport infrastructure (roads, forest roads, trails);
•  the bonfires and burns.

Generally, 1:5,000 scale shots are reserved for urban areas, while 1:15,000 and 1:40,000 scale shots are generally used in the southern part of Quebec, south of the 52nd parallel of latitude, for topographic and forest mapping purposes.

The raw image, as taken on board the aircraft, can be developed and printed on film or slide paper.

The following are the types of aerial photographs most commonly used by departments and agencies for land and resource management:

•  Aerial photographs in black and white at scales of 1:15,000 and 1:40,000

Black and white aerial photography is stored on panchromatic emulsion film (black and white). This emulsion makes it possible to print on photo paper a range of about 50 different shades of grey, which represent different objects on the territory.

• Aerial photographs in color infrared and color at a scale of 1:15,000

- Colour photography can be used for many purposes: forestry, agriculture, land use planning, etc.

When it is digitized and corrected and has the same geometric location properties as a map, it is called orthophotography.

Orthophotography is an aerial photograph:

- which has been scanned digitally in high resolution;

- which has been geometrically corrected and straightened from the ground to eliminate inaccuracies caused by camera tilt at the time of the shooting and due to image distortion caused by terrain;

- to which the coordinates (georeference) have been added;

- which can, therefore, be used in a computer to superimpose other geographical information, measure distances, etc.

Photos of the future...

More and more, aerial photographs are taken directly in digital mode, with digital airborne cameras. These digital airborne images are taken, in the same way as satellite images, by electronic detectors. Therefore, as with satellite images, their accuracy is given in terms of resolution and not scale. In addition, these cameras simultaneously take panchromatic images (in black and white) and color images that are also called multispectral (in red, green and blue). Panchromatic bands are generally of higher resolution.

The great advantage of using airborne digital cameras is that there is no film, so there is no development operation. In addition, the photo does not need to be scanned to produce a digital image and is easier to store. Finally, these images have great qualities, since they are both in good resolution in black and white and in color.

On the other hand, the great advantage of traditional cameras is their wider field (they cover more territory per shot) and their price is much lower than that of large format digital equivalents.

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Why use aerial photographs?

Aerial photography is very rich in information. Among other things, it can be used to identify streams, ponds, trails, and clearings that are not shown on a map. It can also identify bonfires and burns, and determine changes and types of vegetation. However, the undeniable advantage of aerial photography remains the possibility of obtaining a three-dimensional vision of the territory through stereoscopy and thus giving an insight into the relief.

Orthophotography is halfway between aerial photography and topographic mapping. It offers the detail and richness of an aerial photograph, combined with the geometric accuracy of a map. Since it is georeferenced, it can be superimposed with geographic information such as:

•  hypsometry (representation of the relief);
•  toponymy (place names: lakes, rivers, road network);
•  territorial boundaries (administrative boundaries, boundaries of recreational and protected areas).

By their nature, aerial photographs and orthophotographs provide an overview of the territory at a specific time in time. This is why they are used to obtain information about the past or even to plan projects.

Back in the past...

Aerial photography can be used as an archive. Indeed, it allows going back in time by performing the photo-interpretation of a shot taken a few years before.

Here are some examples of the use of old aerial photographs for various fields of activity:

•  justice
•  land-use planning
•  civil security

 justice

When the land on a property is in dispute, old aerial photographs are often used to obtain information on the condition or delimitation of the property before the changes that caused the dispute. In such a case, these photographs become very reliable support for establishing evidence.

land-use planning

Old aerial photographs are often used to redevelop the site as they were in the past. The photographs will then be used to prepare the basic elements of the urban development master plans.

civil security

Aerial photographs can be used to assess the extent of damage caused by a natural disaster.

•  Flooding in the Saguenay
•  The ice storm

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The witness of the evolution...

Aerial photography, when taken in an urban environment, makes it possible to show the stages of urbanization or its spread by the juxtaposition of images captured at different times.

Here are some examples of use for various fields of activity:

•  land-use planning
•  forestry

land-use planning

Aerial photography makes it possible to study the evolution of urbanization over time.

forestry

It is used to obtain information on the physiognomy and groupings of forest species (stand types). It also makes it possible to assess forest potential, since it is possible to inventory forest resources and obtain an approximation of the volume of wood. Thus, the evolution of tree species or cutting areas can be more easily monitored to ensure proper management.

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A vision of the future...

Aerial photographs can be used to conduct site research analyses or plan large-scale projects.

Here are some examples of how to use it for project planning:

•  civil engineering
•  agriculture

civil engineering

Aerial photographs can be used to search for sites (factories, dams, ports, etc.) or to make preliminary network projects (roads, railways, canals, power transmission lines, pipelines, etc.). They can also be used as orthophotographs to take measurements, add layers of information, plan and even make project models.

agriculture

Aerial photographs are used to predict crop yields, estimate crop damage caused by insects, diseases or floods, assess soil water balance, discover areas affected by salinization, etc. They provide good forecasts of agricultural potential.

Structuring consists of verifying the conformity of the data and establishing the relationship between the different elements.

Storage allows geographic information to be stored in databases. These databases store the geometry of the elements as well as the descriptive data related to these elements.

Processing makes it possible to use the geographical database to study a phenomenon, i.e. to analyze it, model it and establish cause-and-effect relationships with a spatial impact.

Dissemination consists of conveying information in the form of raw data, tailor-made products or processing results. Information can be transmitted on paper, such as a map or image, on a digital medium, such as a CD-ROM or a telecommunications process, or on the increasingly used Web.

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The cartography

The cards

Mapping is the art, technique, and science of map-making. It allows the geographical representation of the natural and artificial elements of a territory, such as a road, a river or a lake, in a system of terrestrial coordinates. A map is always presented at a specific scale, which varies according to the detail or portion of territory to be represented.

A map is the projection of a curved surface such as an ellipsoid or a sphere, or part of it, onto a plane. This is called a cartographic projection. Different types are depending on the size and shape of the territory to be mapped. The most commonly used projections in Quebec are the Transverse Mercator (cylindrical projection) and the Lambert Conformal Conic (conical projection).

The use of geographic data and maps is not new. From the beginning of the colony, explorers carried out measurements on the territory to subdivide it into seigneuries, manage port and military infrastructures or locate maritime and land routes.

There are several types of maps that are grouped into two main groups.

Topographic maps contain information:

•  planimetric (hydrography, roads, buildings, vegetation, equipment, place names, etc.); and 
• hypsometric (contour lines and dimensional points).

-Topographic maps are the official cartographic foundation of the Government of Quebec.
 This information, also called reference information, comes from government programs and missions. It gives land managers a common spatial reference. It is to this basic geographical reference that the thematic data specific to each department and agency are added.

-Thematic maps provide information on specific themes such as the road map, land use map (example below), geological map, forest map, etc.

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The scales

A map is always presented at a specific scale, which varies according to the detail or portion of territory to be represented. For example:

•  a large scale map shows a small portion of territory with a lot of detail;
•  a small-scale map shows a large portion of territory with little detail.

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Why use the cards?

The cards can be used to meet a multitude of needs in most areas. A map allows a spatial representation of phenomena on the territory where only imagination remains the limit!

The following are examples of the use of maps in digital or printed form, whether by the professional in the performance of his duties or by the citizen in his travels or the practice of activities on the territory:

•  forestry
•  agriculture
•  hunting and fishing
•  open-air
•  transport

A map is very useful in the forestry sector for planning activities on cutting areas and thus allowing better management of the area and resources.

The maps are used to plan various agricultural operations, locate cultivated and non-cultivated areas and crop types. A map can greatly improve the management of agricultural land.

Hunting and fishing enthusiasts use the map and compass to plan their itinerary and orient themselves in the forest.

Maps are used, for example, to plan a bicycle route that can be followed on a GPS receiver to ensure safe travel.

The road map is undoubtedly the most widely used by citizens for their car journeys on the territory.

makes it possible to collect and record angle and distance measurements in the field.

• Photogrammetry

 uses aerial photographs to interpret, position and measure visible topographic details.

• A survey

 determines the position and boundary of a property, building or administrative territory based on an analysis of previous titles, laws, regulations, and surveys. The operation may include the creation of an official territorial designation and the establishment of ground markers, in order, among other things, to reconstitute at any time the perimeter of the land or territory concerned.

• Remote sensing

collects information on a ground target by analyzing and interpreting images captured from platforms such as satellites.

• Mapping

allows the geographical representation of the natural and artificial elements of territory in a terrestrial coordinate system.

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Why use geomatics?

Over the years, geomatics has become an essential management tool. The Government has several management systems developed from geomatics databases.

The Geographic Information System for Electoral Representation allows the Chief Electoral Officer to make quick decisions on the delimitation of territorial electoral units. It can thus produce different hypotheses by integrating Statistics Canada census data. This example of an application for redistributing territory shows, in black, the boundaries of the 1992 electoral districts and in colored ranges, the grouping of polling subdivisions according to a new delimitation, namely the 2001 electoral districts

Thanks to the implementation of these tools for knowledge of the territory, transportation, and natural resources, managers have new geographic information systems that facilitate decision-making.

Do geomatics!

In order to familiarize yourself with geomatics and help you better understand its various applications, this section provides you with a simulation.

You must use data to solve a social problem, namely the delimitation of electoral districts within a municipality, using Quebec software designed for this purpose. You will be able to see how geomatics can help a project manager make decisions