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
