CE 397 GIS in Water Resources

Projection Information

Datum Conversion

The difference between NAD27 and NAD83 is that NAD 27 does not include Alaskan islands and Hawaii.

To convert from NAD27 to NAD 83

projection stateplane
units feet
zone 3426
datum NAD27 NADCON
parameters
output
projection stateplane
units meters
zone 3426
datum NAD83 NADCON
parameters
end

WGS84 and NAD83 are essentially the same so it is not necessary to convert the datum.

Units

There are a few possible units for geographic projection:

radians
DMS: degrees, minutes, seconds indicated by DD MM SS (e.g. 10 30' 30" is listed as 10 30 30)
D: decimal degrees (e.g. 10.55)
DM: decimal minutes (e.g. 630.5)
DS:decimal seconds (e.g. 37830.0)

Other projections may also have these units:

feet
meters
units per meter (real meter)

Parameters

It is optional to state the datum to be used for all projections except those which support data on a sphere only.

DATUM name{method}
DATUM NONE
DATUM USER_DEFINED dx, dy, dz

The term name is simply means name of the datum. The term method is the datum conversion method used to convert between the specified datum (e.g. NADCON).

The classification none is used when projecting data on a spheroid or sphere to a projection which supports data on a sphere only

You can also specify the spheroid to be used. However, do not use the SPHEROID subcommand if DATUM is stated in the parameters.

The default is Clarke 1866. AN other spheroid which can be used is GRS80. Look in Help "Map Projections " for a complete list of spheroids.

NOTE:ARC/INFO does not convert between spheroids unless transforming between datums.

Projections:

AZIMUTHAL

Here is a list of parameters for azimuthal projection.

projection azimuthal
units: ft or meters
parameters
radius of the sphere of reference
longtitude of center of projection
latitude of center if projection
false easting (meters)
false northing (meters):

The latitude of true scale is usually the Equator, but it doesn't have to be. The radius of the sphere of reference must be a real number. You should note that Azimuthal is supported only on a sphere.

CONICAL

Here is a list of parameters for a conical projection.

projection lambert
units: feet
parameters
1st standard parallel:37 10 00
2^nd standard parallel:37 50 00
central meridian: -123 00 00
latitude of projection's origin:0 00
false easting: 0.0
false northing: 0.0

The longitude west of the Prime Meridian and the latitudes South of the Equator are negative. The false easting and northing are real numbers and can be zero. The latitude of origin defines the origin of the y coordinate for the projection.

CYLINDRICAL

Here is a list of parameters for a cylindrical projection

projection cylindrical
units: feet
xhsift:100000000
yshift:2000000
parameters
longtitude of central meridian:171 0 0
latitude of standard parallel:-44 0 0
azimuth:45.0000
scale factor:1

The longitude west of the Prime Meridian and the latitudes South of the Equator are negative. The azimuth must be entered as an angle in decimal degrees. The scale factor of the central line is a real number, usually one.

ZONE FOR UTM

Each UTM zone has its own central meridian and spans 3 degrees west and 3 degrees east. There are 60 zones. You can look in the HELP in ARC under the Map Projection for a table with all the zones and their spans.

Examples

The following projection file, called alb_lamb.prj, converts from Albers projection to a Lambert Projection

input
projection albers
units meters
spheroid CLARKE1866
parameters
29 30 0.000
45 30 0.000
-96 0 0.000
23 0 0.000
0.00000
0.00000
output
projection lambert
units meters
spheroid GRS1980
parameters
27 25 0.000
34 55 0.000
-100 0 0.000
31 10 0.000
1000000.00000
1000000.00000
end

Another example is the following file, called geostate.prj, which converts from geographic coordinates to stateplane.

input
projection geographic
units ds
zunits meters
datum NAD83
spheroid GRS1980
parameters
output
projection staeplane
units feet
zunits feet
datum NAD27
spheroid CLARKE1866
zone 5376
parameters
END