What is georeferenced information - How is information projected on a map
Georeferenced information has an established location, in terms of a map projection or a coordinate system. See http://en.wikipedia.org/wiki/Georeferencing
It basically consists of data that describe observations or measurements of characteristics of a field. Moving from field-average to site-specific management increases the amount of data that are available for decisions, but also increases the challenge of storing and organizing the data. In their simplest form, data in a GIS are displayed as a map of the area of interest.
As initial considerations when georeferencing farm data:
- In case that you have information not tied to a geographic position, you may need to 'georeference' it. Georeferencing may involve shifting, rotating, scaling, skewing, and in some cases warping, rubber sheeting, or orthorectifying.
- In case that you are collecting new farm data, it's specially important to keep in mind the importance of georeferencing, in order to integrate different layers of information , establish relationships, and draw conclusions. This applies to data that is captured automatically from yield monitors, or application equipment, or manually, such as collecting GPS data during a field visit.
The resulting dataset is commonly known as 'geodata' Information describing the location and attributes of things, including their relationships, shapes and representation. Geographic data is the composite of spatial data and attribute data.
Finally, spatial analysis can be used to link data points to individual points on the maps. Statistics, simulations and models are additional analytical tools that can be applied through the GIS to extract more information from the data to support decisions.
When we start working with farm maps, and georeferencing farm information, we need to take into account what projection will be used.
A map projection is any method of representing the surface of a sphere or other shape on a plane. Map projections are necessary for creating maps, since it's required to transfer information from a model of the Earth, which represents a three-dimensional curved surface, to a two-dimensional medium—paper or a computer screen. See Wikipedia definition of Map Projection for more information.
The earth can be represented by various models, each of which may provide a different set of coordinates (e.g., latitude, longitude, elevation) for any given point on the Earth's surface. The simplest model is to assume the earth is a perfect sphere. As more measurements of the earth have accumulated, the models of the earth have become more sophisticated and more accurate. In fact, there are models that apply to different areas of the earth to provide increased accuracy. (e.g., North American Datum, 1983 - NAD83 - works well in North America, but not in Europe).
When you provide a set of geodetic coordinates to locate something on the surface of the Earth, is also necessary to know the datum of the map in use, which is the reference surface from which measurements are made; if the datum is changed, any selected location can have different geodetic coordinates. This page has more information about datums
Different projections are used for different types of maps because each projection particularly suits certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes.
The figure below illustrates how the earth surface is projected to diferent types of projections.
In agricultural and environmental applications, the State Plane / Universal Transverse Mercator (UTM) types of projection are widely used, as much of the available cartography is developed using such systems.
As in earth models, Projected Coordinate Systems are developed to work well in certain parts of the globe e.g., The NAD 1983 StatePlane Illinois East Zone projected coordinate system does not work well outside its zone of application.
Geographic coordinates / use of Geographic Coordinate System(WGS84)
Today, many people are aware of the use of GPS for locating objects in terms of latitude, longitude and height, applying these concepts in widely adopted applications such as Google Earth, or GPS navigation systems for mobile devices.
These type of coordinates that we frequently see on maps, are geodetic latitude and longitude, which are angular units, the unit of measurement on a sphere or a spheroid, such as degrees. On the other hand, projected maps use planar coordinates, such as feet or meter.
Please keep in mind that, as mentioned above, when geodetic coordinates are provided you need to specify its Datum as well.
GeoAgro GIS uses a Geographic Coordinate System (GCS) to represent maps and the World Geodetic System from 1984 (WGS84) as the Datum, with the purpose of simplifying data management for users:
- this is the reference coordinate system used by the Global Positioning System (GPS), facilitating the exchange of information between the GIS and GPS for data collection.
- by integrating the information in a single projection system, it simplifies the task of working with multiple systems. When information is imported into GeoAgro GIS, it is automatically projected to the common GCS WGS84 system.