Fall 2005

Lab 8: Field Based Floodplain Mapping and GPS Error Assessment

Due Week 11

What is this person doing? He is using GPS to obtain primary field data pertaining to floodplain land cover in eastern Mexico, to be used in a GIS. Working with field data is an important component of environmental GIS, and GPS is a common approach for collecting field data. In this lab you will use GPS to map the Waller Creek floodplain between 24th and 23rd St. Delineating floodplain features in the field is very different from using secondary data sets, such as topographic maps or satellite imagery. In addition to learning the basics of GPS and improving your knowlege of floodplains and rivers, this lab illustates very important concepts related to data quality and GIS. You will evaluate the quality of your GPS data by calculating the root mean square error (RMS) based on the position of the benchmark outside of the Geography Building. Lastly, this lab will introduce to you a very practical procedure for creating GIS data from field data. Upon importing your field data into ArcGIS you will create new shapefiles for the different features. These data will be compared with independent secondary data sources, including the 100 yr FEMA floodplain boundaries.


1. GIS concepts

2. GIS procedures

3. Geographical concepts

Part I

Introduction to GPS

GPS in a Nutshell
Global Positioning Systems (GPS) is a satellite based navigation system for identifying position (X, Y, and Z) on the earth’s surface.  Twenty-eight satellites orbit the earth, ensuring that there are between four and eight satellites (four is the minimum) available at anytime or place from the earth's surface.  Position is determined by measuring the distance from a point on the earth’s surface to the orbiting satellites.

Overview of GPS Technology
GPS was developed, and is controlled, by the Department of Defense (DOD), primarily as a result of the missile buildup during the cold war.  Over 12 billion dollars was spent developing the system, and now GPS is available for the general public to use.  Indeed, GPS has become very popular in the public domain, particularly since the early 90s. The increase in public usage of GPS can be related to several factors, including a reduction in price and an increase in quality, the ease to which the technology can be integrated into environmental data collection and field research, and the DODs commitment to invest billions of dollars in a missile defense strategy.  In fact, it was only a few years ago that the 24th satellite was put into orbit by the Air Force, enabling GPS to be invoked at any time or place on the earth's surface.  However, perhaps the single, all inclusive reason for the growth and proliferation of GPS has been a very fundamental issue that has always confronted human beings: the desire to know our location in space (we, as Geographers can appreciate this!).  This is perhaps the manifest destiny of GPS technology: a single, burning question that has pushed technology in the direction of creating an affordable and easy to use system.  Thus, today the general public can obtain a quality GPS unit for a few hundred dollars.

GPS Satellite System
The modern GPS system consists of twenty-eight (give or take a couple) satellite vehicles (SV) in continuous orbit.  Each SV was carefully put into orbit by the Air Force, at 20,200 km above the earth’s surface.  Additionally, the satellites orbit within six equally spaced orbital planes, with four satellites within each plane.  The high altitude of the satellites greatly reduces the amount of distortion and interference that would occur from the earth’s atmosphere. This insures the SVs stay on track, and follow very predictable orbital paths.  A network of six ground based control stations monitors the health of each SV.  Each control station checks for errors in the SVs orbital paths, which are caused by fluxes in solar radiation and the gravitational pull of the sun and the moon.  Twice daily the control stations transmit an ephemeris code to each SV, which is a model of their orbital path, insuring that the system of SVs are correctly positioned.  Each SV then broadcasts the ephemeris code to the ground based receivers (your hand-held GPS units) with its unique Pseudo Random Noise (PRN) Code.  The ephemeris code may then be used by the GPS receivers, or a PC, for correcting positional errors.

GPS Satellite Signals
The key to GPS is that each satellite transmits its own unique PRN, which allows for each satellite to be identified by the GPS receivers on earth.  The PRN code has a low frequency (similar to radio waves) that does not send very much information.  This allows the PRN to be received by relatively low power receivers, and is one of the features of modern GPS units that make them so portable. The PRN signal is divided into time periods, referred to as chips.  There are two types of PRN codes: the coarse acquisition code (C/A), which is used by civilian receivers, and the P code, a slightly more powerful code that is reserved exclusively for military purposes.  The fundamental distinction between the military and civilian codes is that the C/A code is interfered with by the military in a procedure referred to as Selective Availability (S/A).  S/A creates a time uncertainty within the PRN, and is the single greatest source of error in civilian GPS.  Effectively, it increases the error in position estimates from the order of centimeters to tens of meters.  The main reason cited for S/A is one of defense: the military does not want our enemies to be able to utilize our GPS system to direct missiles at the U.S.

Note: As of May 1st, 2000, the military has turned off S/A. This means that your positional accuracy from the hand-held GPS units increases from +- 100 m to roughly +- 25 m. However, the military reserves the option to activate S/A anytime they deem necessary (in times of crisis).

The unique PRN code is transmitted as time-tagged data bit frames that enable the time of transmission of each PRN code to be exactly determined. Each SV repeatedly transmits its data code at precisely the same time that the other satellites in the system transmit their own unique PRN code.  To insure that the SVs each transmit their satellites at exactly the same time, they are synchronized by a set of atomic clocks. Although only one atomic clock is necessary, each SV carries four atomic clocks to prevent against failure.  In essence, if the atomic clocks were to go bad, the SV would be lost from the system, and that would jeopardize GPS.  Atomic clocks are the most precise clocks ever built. They are not actually fueled by atomic energy; but instead rely on the known oscillation of two particular atoms (cesium and rubidium) as a metronome to keep time.  The atomic clocks are also very expensive, with each costing over $100,000.  The timing of the atomic clocks are continuously monitored and adjusted by the ground-based control stations, which transmit corrections to insure that all of the SVs are synchronized.

Using GPS to Determine Position (X,Y, Z)
Obtaining a position on the earth’s surface with GPS is based on satellite ranging, which means that our position is determined by measuring the distance to a group of moving satellites.  This is made possible because we know the exact altitude of each SV and the time that the PRN was transmitted. In essence, GPS works by measuring how long it takes a radio signal from a satellite traveling at a known rate (186,000 miles per second) to reach our GPS receiver, and then calculating the distance from the time.  Thus, distance is equal to velocity x travel time.  For example, if a car leaves a specific point traveling at 60 miles an hour and travels for two hours it has traveled a distance of 120 miles (60 x 2 = 120 mi).  Therefore, because we know: 1. The exact orbital path of each SV, 2. The speed of the PRN (186,000 mph), and 3. Exactly when it was transmitted from the SV, the distance between each SV and a point on earth can easily be calculated for each unique PRN code received by a our ground-based GPS units.  However, although knowing the distance from a point on the earth’s surface to a single satellite greatly reduces the possibility of where a point could be located, we need several SVs so that we can accurately pinpoint a location. Although theoretically knowing the distance to three SVs allows us to triangulate our position, in practice we need a fourth SV so that we can eliminate the chance for error.

Sources of Error
There are numerous sources of error with GPS.  One of the most significant is from the earth’s ionosphere, a blanket of electrically charged particles 140-200 km above the earth’s surface.  Ionospheric particles slow down the PRN signals transmitted from the SVs. Other sources of error include the atomic clocks (not believed to be a major problem), errors inherent within the GPS unit due to rounding of numbers, and multipath errors. Multipath errors refer to the deflection of the GPS signal as it travels through the lower atmosphere.  Additional sources of error include Geometric Dilution of Precision (GDOP), which causes uncertainty due to the angle PRN code transmitted by the SVs.  A good GPS unit will select the SVs that occur at the optimum angle.  Of course, S/A represents the most troublesome source of error. Collectively, these errors can result in a great deal of uncertainty for GPS measurements, although their are correction procedures which can result in GPS readings having an error of < 1 cm.


1. Dana, P. H., 1999.  Global Positioning System Overview, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder.

2. Hurn, J. 1989.  GPS: A Guide to the Next Utility, Trimble Navigation, Sunnyvale, CA, 76 pp.

3. Trimble Navigation, 1999. All About GPS, http://www.trimble.com/gps/index.htm.

Overview of Procedures

I. Basic operation of the GPS unit

II. GPS error assessment

III. Floodplain mapping in the field and integrating the data in a GIS

IV. Assessment and cartographic output


I. Operation of the Garmin 12 handheld GPS receiver

This is designed to get you familiar with collecting GPS points in the field. Be certain to consider the types of error and problems in collecting GPS points in this environment. The following table contains a description of the functions assigned to the different buttons in the unit:

This key turns the unit on and off. To torn the unit off you have to press and hold the key for about 3 seconds.

This key allows you to scroll through the different pages and menus sequentially.

Use this key to capture a position (Waypoint) and store it in the unit's memory
Use this key to navigate to a certain waypoint.
This key is used to confirm data entry and to select highlighted fields to allow data entry
Returns the display to a previous page and cancels data entry in a field (restores to its previous value)
Use these keys to navigate through the different fields in a page. They also allows to change alphanumerical characters when entering data in a selected field.
Moves the selected character field and moves the field highlight from field to field.

1. To acquire a waypoint

II. GPS Error assessment

1. Take different readings for the same point

As mentioned above, there are many different sources of error involved in the location of features using GPS techniques. With this in mind, you are going to evaluate the overall accuracy of GPS readings over a period of time spanning several days. To do this you will need to revisit the benchmark at different times and acquire a new point each time. Then you will compare these readings with a reference pair of coordinates for the benchmark, and also will calculate a measure of error for your set of readings.


2. GPS error assessment; calculating the root mean square error (RMS) for your GPS points

Error assessment is a necessary component of GIS projects. Although it is easy to obtain a GPS coordinate, it does not mean it is accurate or precise. A commonly reported error statistic for GIS data is the root mean square (RMS) error. The RMS is the square root of the average squared error, and is a measure of dispersion for an entire data set. In this portion of the lab you will use the 15 waypoints you took for the benchmark to measure the amount of error. RMS is calculated by:

  1. Type in also the date and time for each waypoint in a separate column.

III. Field based floodplain Mapping

In this section of the lab you will be mapping a portion of the Waller Creek floodplain between 24th St. and 23rd St.

1. Collecting field data

2. Designing your GIS database

3. Bringing your field data into ArcGIS

4. Convert GPS points to a shapefile and use the field points to digitize the landscape features

IV. Assessment and Cartographic Output


Prepare a four page write up of your floodplain mapping project. Be sure to devote considerable time to the issue of error, GPS, and GIS, taking into account the different types of error as discussed in class lecture, discussed in your text, and in the online reading. Include figures and plots where appropriate... but these do not count towards your page total. Be sure to address the following issues:

Grading criteria

Created 2/1/99 by pfh, modified by mp 03/27/04, last edited by pfh 11/1/'05