Correct Datum Transformations of WGS84 GPS Data into GIS for NAD83 and NAD27 1
WGS84 Geographic
(Original GPS)
D
NA iff. C
D8 orre
3B
c
ase t with
Sta
tion
ith n
t w tatio )
c
rre S PS
Co ase us G
.
f
B
f
Di 84 omo
GS uton
W a
(or
E
NA xport
D83
u
(CO sing
RS
96)
Export using
NAD83 (CORS96)
Solution in terms
of NAD83 3
ing )
us nus
t
r
po (Co
Ex 83
D
NA
NAD83
(Correct)
GIS d
atu
m con
versio
n usin
N
AD
(Use o
g
f NAD CON
CO N
error p
m
a
reviou
sly ind intains
uced)
NAD83
4
(with small error)
is typically much larger than error produced by
WGS84 to NAD83 (~1 meter).
3 Refer to Trimble Support Note "Resolving the NAD83
Datum Transformatin Issue" for details on this process.
NAD27 (NADCON)
GIS data2
using
nversion
o
c
m
tu
a
GIS d
N
NADCO
NAD83
4
(with large error)
G
IS
NA d
D_ atu
19 m
83 con
_ W ve
G rsi
S_ on
19 us
84 in
_5 g
GIS
da
NAD tum conv
_198
3_W ersion us
GS_
1984 ing
_1
4 Error produced from WGS84 to NAD27 Conus, etc.
2 Depending on how/if the position was differentially
corrected, it may be necessary to use NADCON to
transform back to WGS84 before moving to NAD83 to
obtain a more accurate NAD83 GPS position as this
process assumes NAD83 equals WGS84.
GI
Sd
atu
m
c
NA onv
ers
D
(al
ter CON ion
us
na
ing
te)
Solution in terms
3
of WGS84
Ex
p
NA ort
DC us
O ing
N
ny
ing r a
us o m
rt us atu
po on d n)
Ex (C D27 atio
7 A
D2 r N orm
NA the nsf
o tra
Export using
NAD83 (Conus)
1 This document graphically shows a complex issue.
The user may elect to take a simplified approach and
assume that WGS84 is equal to NAD83 and accept
the ~1 meter difference between the two datums.
WGS84 Geographic
(Converted GIS data)
NAD27 (Conus, etc.)
GIS data
g
sin _4 ion
u
4
n 8
at ,
sio _19 orm D27
r
)
e
f
nv GS ns NA S
co _W tra t to ONU
r C
tum _to tum ve
da 927 da con s to
S
c
ie
GI D_1 ecifi d to ppl
a
A
e
p
N e s us #4
h
ly se
r t al
(o igin is ca
or th
in
Mark D. Roper
GIS Specialist
San Juan National Forest
10/12/2005