Geodesy and DatumsGeodesy and Datums

Michael A. McAdamsMichael A. McAdamsGeography Dept.Geography Dept.Fatih UniversityFatih UniversityIstanbul, TurkeyIstanbul, Turkey

Is the earth round?Is the earth round?

Well...all the globes are round..so it Well...all the globes are round..so it must be...must be...

It must be flat because..I have never It must be flat because..I have never seen the earth curveseen the earth curve

It is not flat, but more like an elispse It is not flat, but more like an elispse It depends on what you mean by It depends on what you mean by

round. It is a sphere, but not a round. It is a sphere, but not a perfect one.perfect one.

A View from Space (Google Earth)A View from Space (Google Earth)

Another view of the earthAnother view of the earth

Türkiye

Istanbul from SpaceIstanbul from Space

You are here

Fatih UniversityFatih University

You are here

Another image of Another image of Istanbul (infra-red)Istanbul (infra-red)

Some questions?Some questions?

Geodesy is the science of measuring and monitoring the Geodesy is the science of measuring and monitoring the size and shape of the Earth and the location of points on its size and shape of the Earth and the location of points on its surface. surface. (NOAA website: (NOAA website: http://www.oceanservice.noaa.gov/education/kits/geodesy/http://www.oceanservice.noaa.gov/education/kits/geodesy/welcome.htmlwelcome.html ) )

Why is it used?Why is it used? To obtain accurate measures on the surface of the earthTo obtain accurate measures on the surface of the earth Who uses it?Who uses it? Surveyers, remote sensing specialists, GIS technicians, Surveyers, remote sensing specialists, GIS technicians,

geographers, archeologists, anyone concerned with spatial geographers, archeologists, anyone concerned with spatial analysisanalysis

What is the difference between a geod and a datum?What is the difference between a geod and a datum? A datum is the specifications to construct a geod or at least A datum is the specifications to construct a geod or at least

a part of onea part of one

The Elements of Geodesy: The The Elements of Geodesy: The Figure of the EarthFigure of the Earth

The Earth's shape is nearly spherical, with a The Earth's shape is nearly spherical, with a radius of about 3,963 miles (6,378 km), and its radius of about 3,963 miles (6,378 km), and its surface is very irregular. surface is very irregular.

Mountains and valleys make actually measuring Mountains and valleys make actually measuring this surface impossible because an infinite this surface impossible because an infinite amount of data would be needed.amount of data would be needed.

It would take you many lifetimes to measure It would take you many lifetimes to measure every crevice, valley, and rise. You could never every crevice, valley, and rise. You could never complete the project because it would take too complete the project because it would take too long.long.

http://www.oceanservice.noaa.gov/education/kits/http://www.oceanservice.noaa.gov/education/kits/geodesy/geo03_figure.htmlgeodesy/geo03_figure.html

Members of a 1922 geodetic survey expedition. Until recent advances in satellite technology, namely the creation of the Global Positioning System (GPS), geodetic surveying was an arduous task best suited to individuals with strong constitutions, and a sense of adventure.

Different Abstractions of the EarthDifferent Abstractions of the Earth

http://www.oceanservice.noaa.gov/education/kits/http://www.oceanservice.noaa.gov/education/kits/geodesy/media/supp_geo03a.htmlgeodesy/media/supp_geo03a.html

Abstractions of the Earth’s surfaceAbstractions of the Earth’s surface

SphereSphereSimular to a ball or what we think of as a Simular to a ball or what we think of as a globe. It discounts the irregularities in the globe. It discounts the irregularities in the earth’s surface such as mountaing ranges earth’s surface such as mountaing ranges and canyonsand canyons

ElipsoidElipsoidLooks like an egg with a bulge in the Looks like an egg with a bulge in the middle. More accurate than a sphere. middle. More accurate than a sphere.

GeodGeodBased on the elevation and earth’s Based on the elevation and earth’s gravatational pull. It is the most accurate gravatational pull. It is the most accurate description of the surface of the earth.description of the surface of the earth.

What is a datum?What is a datum?

AA datum is as a set of spatial information datum is as a set of spatial information that acts as a foundation for other data, that acts as a foundation for other data, just like concrete acts as a foundation for just like concrete acts as a foundation for the structure of a building, including all the structure of a building, including all the furnishings and decorations inside.the furnishings and decorations inside.

Without a datum there would be nothing Without a datum there would be nothing to securely support other spatial to securely support other spatial information, such as digital elevations, information, such as digital elevations, land use, or population. land use, or population.

There are two types of datums, horizontal There are two types of datums, horizontal and verticaland vertical

Horizonal and Vertical datumsHorizonal and Vertical datums

the horizontal datum is a collection of specific the horizontal datum is a collection of specific points on the Earth that have been identified points on the Earth that have been identified according to their precise northerly or southerly according to their precise northerly or southerly location (latitude) and easterly or westerly location (latitude) and easterly or westerly location (longitude).location (longitude).

The vertical datum is a collection of specific The vertical datum is a collection of specific points on the Earth with known heights either points on the Earth with known heights either above or below mean sea level. Near coastal above or below mean sea level. Near coastal areas, mean sea level is determined with a tide areas, mean sea level is determined with a tide gauge. In areas far away from the shore, mean gauge. In areas far away from the shore, mean sea level is determined by the shape of the geoid.sea level is determined by the shape of the geoid.

http://www.oceanservice.noaa.gov/education/kits/geodesy/

Global Positioning System (GPS)Global Positioning System (GPS)

Using the Global Positioning System (GPS), every point on Earth Using the Global Positioning System (GPS), every point on Earth can be given its own unique address -- its latitude, longitude, and can be given its own unique address -- its latitude, longitude, and height. The U.S. Department of Defense developed GPS satellites height. The U.S. Department of Defense developed GPS satellites as a strategic system in 1978. But now, anyone can gather data as a strategic system in 1978. But now, anyone can gather data from them.from them.

GPS is a constellation of satellites that orbit approximately 11,000 GPS is a constellation of satellites that orbit approximately 11,000 miles above the Earth and transmit radio wave signals to receivers miles above the Earth and transmit radio wave signals to receivers across the planet. By determining the time that it takes for a GPS across the planet. By determining the time that it takes for a GPS satellite signal to reach your receiver, you can calculate your satellite signal to reach your receiver, you can calculate your distance to the satellite and figure out your exact location on the distance to the satellite and figure out your exact location on the Earth. Sound easy? In fact it is a very complicated process. For the Earth. Sound easy? In fact it is a very complicated process. For the GPS system to work, you need to have incredibly precise clocks on GPS system to work, you need to have incredibly precise clocks on the satellites and receivers, and you must be able to access and the satellites and receivers, and you must be able to access and interpret the signals from several orbiting satellites interpret the signals from several orbiting satellites simultaneously. Fortunately, the receivers take care of all the simultaneously. Fortunately, the receivers take care of all the calculations. calculations.

http://www.oceanservice.noaa.gov/education/kits/geodesy/http://www.oceanservice.noaa.gov/education/kits/geodesy/geo09_gps.htmlgeo09_gps.html

ConclusionConclusion Geodesy is an important for accurate measurements of the Geodesy is an important for accurate measurements of the

earthearth Geods, elipzoids and spheres are abstractions of the earthGeods, elipzoids and spheres are abstractions of the earth Datums are the measurements or data that directs the Datums are the measurements or data that directs the

contruction of an elipzoid, sphere or geodcontruction of an elipzoid, sphere or geod If accuracy is important (i.e., GPS)If accuracy is important (i.e., GPS)the mixing or non-transformation of datums can be perhaps the mixing or non-transformation of datums can be perhaps

the difference between life or death if using it for navigation the difference between life or death if using it for navigation purposespurposes

Datums, projections and coordinate systems are related Datums, projections and coordinate systems are related issues, as will be discussed in the next lectureissues, as will be discussed in the next lecture

Some Other ReferencesSome Other References

Datums, who needs ‘em anyway?Datums, who needs ‘em anyway?http://www.esri.com/news/arcuser/0401/dahttp://www.esri.com/news/arcuser/0401/datum.htmltum.html

Geodesy Overview by Peter Dana:Geodesy Overview by Peter Dana:http://www.colorado.edu/geography/gcrafthttp://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html/notes/datum/datum_f.html

Geodesy Tutorial by NOAA:Geodesy Tutorial by NOAA:http://www.oceanservice.noaa.gov/educatihttp://www.oceanservice.noaa.gov/education/kits/geodesy/welcome.htmlon/kits/geodesy/welcome.html