Note:
this information updated November 17, 2004.
Data Accuracy
GPS accuracy ranges from less than 1 cm
to more than 100 m, depending on the equipment and techniques being used. In
addition to errors related to satellite geometry (see Mission
Planning), there are clock errors, atmospheric delay errors, multipath
errors and selective availability errors. Selective availability (S/A) is
usually the largest source of error in GPS data - this error is deliberately
introduced into satellite signals by the DOD to deny unauthorized users access
to the strategic advantages of GPS. S/A often results in data that is accurate
only to about 100 m (this is horizontal accuracy - vertical accuracy is usually
2 to 5 times worse than horizontal accuracy) (Figure 1). (Update 4/20/01: S/A was turned off in May of 2000.
This has resulted in significant improvements in GPS accuracy. The GeoExplorer
has an error of 20-30 m for uncorrected data and 2-5 m for differentially
corrected data).
Figure 1. Uncorrected GPS map of a
rectangular parking lot (approx. 70x90 m).
The effect of S/A is clearly apparent - each position mapped along the
perimeter of the parking lot has an
error of up to 100 m. Data collected
on June 25th, 1999, between 2.32 p.m. and 2.57 p.m.
For some mapping purposes 100 m
errors may be acceptable - for example, mapping parking lots as points in a
large city. However, for many mapping projects 100 m errors will not be
acceptable. Fortunately, much of the error in GPS data can be corrected by a
technique called differential correction. Differential correction is
based on collecting a set of base files on a receiver (base station) at
a precisely known location (reference position) at the same time as the rover
files are collected. The base files must contain data from the same satellites
as the rover files - in effect this means that the rover files must be
collected within 300 miles of the base station (so that the rover receiver and
the base station receiver are "seeing" the same set of satellites).
The data collected at the base
station is used to calculate the error in the satellite signals (by finding the
difference between the positions calculated from the satellite signals
and the known reference position) - the resulting differential corrections
can be used to remove much of the error from the rover file data. After
differential correction, the GeoExplorer provides accuracy typically on
the order of 2 to 5 m. It is possible to obtain real-time differential
correction - this means that a radio signal containing the differential
corrections is transmitted from a base station to a receiver connected to the
GPS unit - the radio signal is used to correct the GPS positions in the field
in real time. However, this technique is not currently available in CSAM
- the alternative, postprocessing differential correction (meaning that
the rover files are downloaded to a PC and then differentially corrected using
base station files), is discussed below.
Base Files
In the U.S, a number of government
and private agencies make base files freely available on the internet and under
normal circumstances coverage can be obtained for almost anywhere in the
country. In Texas, the Department of
Transportation (TXDOT) provides base files from 27 base stations within the
state (including Arlington)
- referred to as Regional Reference Points (RRP's) by TXDOT. Base files are
kept for 30 days. There are 4 base files for each day, each covering 6 hours.
The files are in a compressed format (zip files). To obtain TXDOT base files,
use a web browser to go to location ftp://ftp.dot.state.tx.us/pub/txdot-info/isd/gps/
(Figure 2).
Figure 2. The Texas Department of
Transportation GPS data ftp site.
For this example, we will obtain a
base file for November 15th, 2004, 6 am – 12 noon. The files are in
the RefData.04 folder (Figure 2). Opening the folder displays month folders
(Figure 3).
Figure 3. Texas Department of Transportation GPS data month folders.
Opening the Month.Nov folder
displays day folders (Figure 4).
Figure 4. Texas Department of Transportation GPS data day folders.
Opening the Day.15 folder displays
station folders (Figure 5).
Figure 5. Texas Department of Transportation GPS base station folders.
Opening the txar folder displays
base station data for Arlington
(Figure 6).
Figure 6. Texas Department of Transportation GPS base station data for Arlington,
November 15th, 2004.
Trimble’s
data file format is used for raw GPS observations. DAT files are produced by
survey grade receivers. This file type has a .dat extension. The four data
files are in .zip folders (compressed).
File
naming convention is based on RRP Station ID, Julian Date, and UTC time, for
example:
txcc171G.dat.zip
txcc
= RRP ID for Corpus Christi
171
= Julian Day for June 19
G =
UTC hour for 1:00am (daylight saving time)
.dat
= format Trimble raw
.zip
= compression
Station
ID
TxDOT
uses the four (4) letter NGS naming standard for station ID. Examples include:
txam = Amarillo
txan = San
Antonio
txar = Arlington
txau = Austin
Julian Day
The Julian calendar is the 365-day
calendar. This calendar is provided in MS WORD format by TxDOT in the documents
folder (Figure 2). November 15th is day 320.
UTC
Time Conversions
The
Coordinated Universal Time (UTC) system is based on Greenwich Mean Time (GMT).
All time zones are computed relative to UTC. For example, in the United
States, Central Standard Time (CST) is 6 hours
earlier than UTC time -- 10:00 UTC is 4:00 CST. Conversions from UTC to US Central Standard Time and Central
Daylight Savings Time are shown below:
Conversions from UTC to US Central Standard
Time:
* = previous day
|
UTC
(GMT)
|
CENTRAL
STANDARD
|
|
00
= A
|
6
pm *
|
|
01
= B
|
7
pm *
|
|
02
= C
|
8
pm *
|
|
03
= D
|
9
pm *
|
|
04
= E
|
10
pm *
|
|
05
= F
|
11
pm *
|
|
06
= G
|
12
mid
|
|
07
= H
|
1
am
|
|
08
= I
|
2
am
|
|
09
= J
|
3
am
|
|
10
= K
|
4
am
|
|
11
= L
|
5
am
|
|
12
= M
|
6
am
|
|
13
= N
|
7
am
|
|
14
= O
|
8
am
|
|
15
= P
|
9
am
|
|
16
= Q
|
10
am
|
|
17
= R
|
11
am
|
|
18
= S
|
12
noon
|
Standard time starts at 2 a.m. on
the last Sunday of October until the first Sunday of April.
Conversions from UTC to US Central Daylight
Time:
* = previous day
|
UTC
(GMT)
|
CENTRAL
DAYLIGHT
|
|
00
= A
|
7
pm *
|
|
01
= B
|
8
pm *
|
|
02
= C
|
9
pm *
|
|
03
= D
|
10
pm *
|
|
04
= E
|
11
pm *
|
|
05
= F
|
12
mid
|
|
06
= G
|
1
am
|
|
07
= H
|
2
am
|
|
08
= I
|
3
am
|
|
09
= J
|
4
am
|
|
10
= K
|
5
am
|
|
11
= L
|
6
am.. and so on
|
Daylight Saving Time begins at 2
a.m. on the first Sunday of April and ends at 2 a.m. on the last Sunday of
October.
The required file is
txar320M.dat.zip (M = the 6-hour period 6 am – 12 noon).
Double-clicking the file will
download it (Figure 7).
Figure 7. File download.
The downloaded zip file can be
extracted with a standard file decompression utility such as WINZIP (Figure 8).
Figure 8. The unzipped DAT file.
Differential Correction
To differentially correct the
parking lot map shown in Figure 1, an older base file (ARL1999062513.dat – note
a different file naming system was used in 1999) collected on June 25th,
1999, must be used. In Pathfinder Office, select Utilities|Differential
Correction - the Differential Correction Utility appears (Figure 9).
Figure 9. The Differential Correction Utility window.
In the Rover Files group,
browse to and select your rover file (c062519a.ssf in this example). In the Base
Files group, click local search to search c:\pfdata\base for your base files. Select
your base file (ARL1999062513.dat in this example) and the Confirm Selected
Base Files window appears (Figure 10) (Note: in this example, the rover
file was collected in UTC (the 19th hour), the base file name is based on CDT
(the 13th hour - these are equivalent, both refer to 1 p.m. local time).
Figure 10. The Confirm Selected Base Files window.
This window shows the selected rover file and base file and confirms that the
entire rover file is covered by the base file (coverage = full).
In the Corrected Files
group, browse to and select your output folder (c:\pfdata\Campus Map in this
example). The file extension for the corrected files is by default
".cor". Under normal circumstances, processing of GeoExplorer
rover files is by code processing only. Note that after the base file is
selected, the Reference Position window automatically opens (Figure 11). This
window displays the reference position of the base station being used (TXDOT
publishes these reference positions on its web site, enabling you to check and
confirm this information).
Figure 11. The Reference Position
window.
Clicking OK on the
Differential Correction Utility window (Figure 9) starts the correction
process. When differential correction is completed, the notification window
appears (Figure 12).
Figure 12. The Differential
Correction Completed window.
The message indicates that 100% of
the 894 positions in the file were successfully corrected. The corrected file
will be in the Campus Map directory and will be named c062519a.cor. The
corrected parking lot map is shown in Figure 13. The general shape of the
parking lot is correct (the large indentations are tree stands). The effect of
the 2-5 m error is apparent in the jagged quality of some lines - this is most
noticeable in the upper right perimeter of the parking lot.
Figure 13. The Corrected Parking Lot
Map (left). The uncorrected map (right) is shown for comparison.