Projecting a data set Defining a projection is very simply creating the text

# Projecting a data set defining a projection is very

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Projecting (a data set) Defining a projection is very simply creating the text file that describes the Spatial Reference of the data. Projecting a data set is either: Transform ing from Geographic Coordinates to Projected (cartesian) coordinates; Transform ing from one projection (e.g. UTM to another (e.g. Teale Albers)( reprojection ).
How changing projections in ArcGIS works!
ArcCatalog is used to define projections only! ArcToolbox can be used to define or project data – only ArcToolbox can change (project and transform) the actual coordinates as stored in the data layer. A common error is to think a projection can be changed using ArcCatalog. – Only the projection description (text file containing projection info) can be changed in ArcCatalog, not the projection of the physical data itself . Defining a Projection (for a data set) vs. Projecting (a data set)
ArcMap “Project-on-the-ﬂy” First layer added to ArcMap defines the data frame’s projection and coordinate system parameters. When additional data is added to the map with a different coordinate system, ArcMap can project this data " on the fly ” into the data frame’s projection. ArcMap can automatically transform the data's projection system to be the same as that used for the map (without physically changing data). Note : If any part of the projection definition of new data added does not match the data frame’s projection parameters then it may not spatially align properly.
Project on the fly First layer added has UTM, Zone 10N, NAD 1983
Project on the fly First layer define Coordinate System of Data Frame as UTM, Zone 10N, NAD 1983
Project on the fly Additional layers with different coordinate systems are shown in UTM, Zone 10N, NAD 1983
44 The 7 possible states of GIS (geospatial) data in regards to projection and definition 1. Projected (e.g. Albers, UTM) and defined correctly 2. Projected (e.g. Albers, UTM) and defined, incorrectly 3. Projected (e.g. Albers, UTM) and un defined A missing *.prj file 4. Not-projected (e.g. GeoDD) and defined, correctly 5. Not-projected (e.g. GeoDD) and defined, incorrectly 6. Not-projected (e.g. GeoDD) and un defined A missing *.prj file 7. Not-projected (e.g. in page units such as inches) and not defined A scan of a paper map or an image file of a map (e.g. *.jpg)
Projections you should know in this class! Albers (typically with NAD83 datum) à Equal-area projection UTM (NAD83 datum, zone 10 & 11 for CA) à Distortion of scale, distance, direction and area increase away from the central meridian GCS (WGS84) à geographic coordinate system in DMS or DD State Plane à use only to project area(s) within one zone California (Teale) Albers (US Feet or Meters)
Coming next…. Geodatabases, Attribute Data, Tables