Projecting (a data set)•Defining a projection is very simplycreating thetext filethat describes the Spatial Reference of the data.•Projecting a data set is either:–Transforming from Geographic Coordinates to Projected (cartesian) coordinates;–Transforming from one projection (e.g. UTM to another (e.g. Teale Albers)(reprojection).
How changing projections in ArcGIS works!
•ArcCatalogis used to define projectionsonly!•ArcToolboxcan be used to defineor projectdata –only ArcToolbox can change (project and transform)the actual coordinates as stored in the datalayer.•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 projectionand coordinate system parameters.•When additional data is added to the map with a different coordinate system, ArcMap can projectthis 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 flyFirst layer added has UTM, Zone 10N, NAD 1983
Project on the flyFirst layer define Coordinate System of Data Frame as UTM, Zone 10N, NAD 1983
Project on the flyAdditional layers with different coordinate systems are shown in UTM, Zone 10N, NAD 1983
44The 7 possible states of GIS (geospatial) data in regards to projection and definition1.Projected (e.g. Albers, UTM) and defined correctly2.Projected (e.g. Albers, UTM) and defined, incorrectly3.Projected (e.g. Albers, UTM) and undefined•A missing *.prj file4.Not-projected (e.g. GeoDD) and defined, correctly5.Not-projected (e.g. GeoDD) and defined, incorrectly6.Not-projected (e.g. GeoDD) and undefined•A missing *.prj file7.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)àEqual-area projection•UTM (NAD83 datum, zone 10 & 11 for CA)àDistortion of scale, distance, direction and area increase away from the central meridian•GCS (WGS84) àgeographic coordinate system in DMS or DD•State Plane àuse only to project area(s) within one zone•California (Teale) Albers (US Feet or Meters)