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RETTEW Associates, Inc
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As a Survey Group Manager at RETTEW, Mr. Jensen has more than 36 years of experience providing surveying services to public and private clients. He has performed various surveying tasks, site plans, structural drafting including connection details and shop drawing review, and quality control for numerous projects in the residential and education markets, among others. His expertise includes construction stakeout, photo control, cost estimating, ALTA surveys, GPS control, topographic.
Natural Lands Trust
610-353-5640 ext. 217
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Megan Boatright, GIS Analyst for Natural Lands Trust, completed her M.A. in Geography at West Chester University in 2009. Using GPS and GIS, she creates maps for conservation easements, land acquisitions, and municipal planning.
Michael S. Batcher, Consulting
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I use GPS coupled with ARCPAD to collect field data and incorporate that into GIS projects.
Featured Library Items
GPS for Land Surveyors, 3rd Edition
Completely revised and updated, this third edition reflects the increased importance of GPS in the everyday practice of surveyors and mappers. Providing an accessible yet rigorous introduction to the subject, the book offers a practical guide to the techniques used in real GPS surveying. It explo...
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A Guide to Understanding Land Surveys
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Material was researched and written by Nicole Faraguna. Some material was excerpted and/or adapted from the Land Trust GIS, http://landtrustgis.onenw.org. Peer review conducted by Megan Boatright, Natural Lands Trust and David Jensen of Rettew Associates.
Nothing contained in this or any other document available at ConservationTools.org is intended to be relied upon as legal advice. The authors disclaim any attorney-client relationship with anyone to whom this document is furnished. Nothing contained in this document is intended to be used, and cannot be used, for the purpose of (i) avoiding penalties under the Internal Revenue Code or (ii) promoting, marketing or recommending to any person any transaction or matter addressed in this document.
Copyright © is held by the Pennsylvania Land Trust Association
Text may be excerpted and reproduced with acknowledgement of ConservationTools.org and the Pennsylvania Land Trust Association.
GPS (the Global Positioning System) can be used freely by anyone almost anywhere near the earth. GPS enables users to record the location of structures, easement boundaries, where a photograph is taken, and other geographic information useful to conservation. Users can then import this information into computer mapping programs.
GPS can be useful for conservation organizations and local governments for monitoring their conservation easement holdings and managing their land preserves. GPS enables users to record positional data in the form of points (e.g., location of a tree or property corner), lines (e.g., a trail) or areas (e.g., a lake). Users can record the locations of any property features that they consider useful for conservation planning such as wildlife habitat units, fences, topography, soil erosion problems, invasive species and easement violations. The recorded data can be imported into Geographical Information System (GIS) software, enabling users to create highly accurate maps of their conservation holdings. Users can also use GPS to quickly re-identify in the field the precise location of previously recorded features.
- GPS technology provides positioning and navigation information for a variety of markets and purposes. Conservation organizations and government agencies are using GPS technology to revolutionize their field and land management practices.
- GPS technology is widely used for engineering and land surveying purposes, with a line of survey-grade equipment developed to ensure accurate data collection.
- GPS typically functions in all weather conditions, although obstructions resulting from tree coverage, buildings or other objects may result in a GPS unit’s inability to receive signals. Weather or atmospheric conditions may cause miscalculations which can be problematic when precise accuracy is essential (e.g., land surveyors).
- Depending on the needs of the organization, a variety of GPS units exist that vary in cost and in the level of accuracy, enabling organizations to budget accordingly and purchase equipment that is right for them.
- Many GPS units are now integrated with GIS and/or Google Earth.
Typical End Users
Users from conservation organizations to local governments to individuals and businesses find GPS useful.
- GPS enables conservation organizations and municipalities to easily and effectively collect a variety of data on their properties and easement holdings that aids in monitoring, mapping and planning.
- GPS can identify precise locations of specific field elements that the organization may need to monitor and revisit (e.g., invasive species, endangered species, easement violations, structures, etc).
- GPS provides the opportunity for accurate data sharing within the organization.
What You'll Need
- A GPS receiver
- At least one staff member skilled in using the unit and software
- Software and skills to download the data from your GPS unit and insert into your GIS system.
Obstacles and Challenges
- For this technology to be valuable to an organization, the organization must invest in a qualified staff member properly skilled in using the equipment and software.
- GPS does not perform well or at all underground, under water or under dense tree coverage, which eliminates the ability for users to collect data in dense forests or caves.
Contents of Main DescriptionWhat is GPS?
Choosing a GPS Receiver
Types of GPS Receivers
Basic or Recreational units
Considerations for Purchasing
GPS Conservation Applications
Prioritization and Planning
Locating and Relocation Features in the Field
What is GPS?
GPS (the Global Positioning System) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. These satellites are positioned about 12,000 miles above the earth’s surface and travel approximately 7,000 miles per hour. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.
GPS satellites circle the earth twice a day in a very precise orbit and constantly transmit coded radio signals to earth. The satellites, which are solar-powered with battery backup for when passing through earth’s shadow, are positioned so that at least four of their signals can reach any given point on earth at one time. These signals can pass through clouds, glass and plastic. The signal weakens when passing through solid objects such as buildings and cannot pass through objects that contain high levels of metals. Specifically for land conservation purposes, it is important to note that a GPS unit will not receive satellite signals when under thick forest canopies, underground, or underwater.
The satellites broadcast signals containing the following information:
- Which satellite is sending the information;
- The exact location of the satellite at any given time;
- Whether the satellite is working properly;
- The date and time that the signal was sent.
GPS receivers take this information and use pseudo range measurements to determine the user's position . Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. As it receives distance measurements from additional satellites, the receiver can calculate the user's position from where the pseudo-ranges from the set of satellites intersect.
A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.
Although GPS was originally developed for military purposes, today the technology provides positioning, velocity and navigation information for a variety of markets and purposes. Anyone who needs to know the precise time or the location of people or objects will benefit from GPS. This information can be used in charting and mapping, plotting a course, navigating from point to point, tracking vehicle movement, locating previously identified sites or any number of similar functions. Marine GPS receivers are proving to be invaluable aids in both recreational and commercial boating and sailing. Land-based professions from surveying to exploration employ GPS. Surveyors, natural resource managers, wildlife managers, geologist, geographers, cartographers , forestry managers, search and rescue teams, public safety professionals, heavy machine operators, archaeologists, utility managers, and oil, gas and mineral explorers are just some of the people taking advantage of GPS. Other applications include avionics and outdoor recreational activities such as backpacking, camping, hiking, mountain biking and snowmobiling.
Choosing a GPS Receiver
A GPS receiver is the electronic device that receives the signals from the satellite system. The user will employ the receiver to collect positional data and then download the data to mapping software. GPS devices range in price and quality.
Technology is always improving so it’s good practice to visit a few websites that evaluate current receivers based on their usability, performance, and reliability before making a purchase.
Some equipment sales/distributor companies rent by the day, which enables organizations to test a unit before purchasing.
GPS units vary in accuracy and sophistication of technology so before purchasing, users should determine their specific needs. Some users may be able to get away with using basic or recreational units that are not designed for pinpoint accuracy. However, some users may require far more accurate placement of features often to within a meter or less. Although some basic or recreational products are not specifically designed for GIS mapping, they can be used successfully in some applications. And for some GIS users, the basic GPS receivers may offer the most cost-effective solution. In helping to choose a GPS receiver, users should review the following questions to determine application needs:
- Will your organization need to integrate data with GIS? If you will be converting GPS points to a specific GIS format, such as shape file format, you want to ensure that the unit you purchase has this capacity. Some units can convert points to popular GIS formats on the fly during downloading.
- Will your organization be collecting attributes along with location points? Many GIS users have found that accurate attribute collection is just as crucial as location acquisition. Only the professional GPS products offer customizable interfaces and routines for detailed attribute collection.
- Is five-meter accuracy sufficient for your application? Basic GPS units may typically achieve 10-meter accuracy, while some are capable of sharpening accuracy to five meters or better. More expensive, professional-grade units can hone in within centimeters and offer a much higher level of accuracy, though for many applications this level of accuracy is not necessary.
Types of GPS Receivers
Basic or Recreational units
These units are the least expensive type of units and only provide basic latitude and longitude coordinates while plotting points of interest and straight-line routes without the convenience of detailed maps. For organizations just starting out, Land Trust GIS recommends Garmin units because of the free software utility available for converting data from your unit to a GIS application created by the Minnesota Department of Natural Resources. This extension enables users to download collected GPS data into a GIS application or upload existing GIS data into a GPS unit. Basic Garmin units cost anywhere between $250 - $1000 (2009 figures). As referenced above, basic units offer sub-par navigation, have limited mapping features, and limit the amount of attribute information to be collected on a specific locality to a short name and ID number. Additional attributes that the user wishes to save must be recorded manually and then inputted into GIS.
These units are more sophisticated and depict real land and sea maps, street maps, topography, and businesses. They provide for more enhanced data collection, map viewing in the field, and precise navigation. A variety of models are available. As an effective yet economical solution, Land Trust GIS suggests using a Personal Digital Assistant (PDA - for example, the HP IPAQ) and a plug-in GPS receiver, the user can gather more extensive attribute information. Combined with a program like ESRI's ArcPad, this system functions as a robust GIS allowing users to have immediate access to field data for GIS use. This whole package will cost about $1,000- $1,500 per unit but is considerably more useful than the basic option. Once set up, users can upload aerial or satellite imagery onto a PDA with any other vector data. For example, a user conducting a site visit of a trail will have the ability to view an aerial image of the landscape while walking along the trail, all while collecting information on the width and length of the trail, erosion problems, and other critical characteristics.
If a conservation organization is collecting large amounts of complex data and very high accuracy is a priority, Land Trust GIS recommends a more sophisticated solution. At this level an organization would purchase a GPS unit that does everything the PDA option offers except it is integrated into one unit. Land Trust GIS acknowledges Trimble, Magellan and Leica as the industry leaders, as they offer a wide suite of different units that range from $2,500 - $15,000. These units come fully loaded with their own proprietary internal software for complex data collection and integration with GIS, a good sized screen for viewing GIS data on your unit while in the field, alpha-numeric keypads for easily typing in information about features, and post-processing software for producing locationally highly accurate data. They may also be available for rent from providers in your area.
These units are sophisticated units that provide horizontal accuracy to within a centimeter. These units are used primarily for activities when this type of accuracy is essential such as highway construction and other engineering projects. The units can cost up to $40,000 and require extensive training and expertise, making them impractical for general resource management applications.
Considerations for Purchasing
Several factors to consider when purchasing a unit:
- The receiver should have Wide Area Augmentation System (WAAS), which ensures accuracy within 3 meters when you receive WAAS in the field.
- Purchase a receiver that offers a multi-channel design, which enables the GPS to lock in your position faster and more accurately, with easier navigation, working even in tougher terrains.
- Pay close attention to the memory capacity of the unit. There is an advantage to purchasing a unit that uses auxiliary memory cards so the user can purchase additional memory capacity in the future if necessary.
- Since the receiver will be used outdoors, the conservation organization will need a rugged piece of equipment that is either water resistant or water-proof. Unfortunately, there is no universal definition of these terms in regards to the specific conditions that these electronic devices can or cannot withstand. Some manufacturers will provide specific conditions under which their equipment will best perform. When choosing the right unit, a conservation organization should consider the conditions under which the device will be used to ensure it is making a good investment.
- Consider the life and capacity of the unit’s batteries. Some units may simply use standard rechargeable batteries, while others will provide a manufactured battery unique to the device. Units vary in regards to how long the battery may last on a given outing. When working in the field, it is good practice to have extra batteries. A device that includes a sleep mode or battery saver function will save on battery usage.
- Keep in mind that as GPS units continue to improve and the technology continues to advance, price of a given unit may decrease dramatically over time.
There are a variety of GPS software packages available on the market, many of which are available for free. Listed below are a few of the more prominent applications.
ArcPad is designed for GIS professionals who require GIS capabilities in the field. It provides field-based personnel with the ability to capture, edit, analyze, and display geographic information easily and efficiently.
Garmin MapSource is a free mapping software that comes with newer Garmin GPS units. Garmin offers free updates to registered users.
Trimble® GPS Pathfinder® Office software supports all aspects of GIS data collection and data maintenance for Trimble GPS Pathfinder receivers and GeoExplorer® series handhelds.
Google Earth is a virtual globe, map and geographic information program. The software is downloaded by the user and uses universal data stored on an online server. It maps the Earth by the superimposition of images obtained from satellite imagery, aerial photography and GIS 3D globe. It is available under two different licenses: Google Earth, a free version with limited functionality and Google Earth Pro ($400 – one year subscription), which is intended for commercial use. Intended for personal, non-commercial use, the free version of Google Earth provides users with interactive, 3D exploration of the planet through terabytes of aerial and satellite imagery. For professional and commercial uses, Google Earth Pro was developed as a research, presentation, and collaboration tool for geographic information. Licensed users of the professional version can import waypoint and track data collected through GPS units into the software though the program currently only supports devices manufactured by Garmin and Magellan. Google Earth can be used with Windows, Mac OS and LInux platforms. A user’s guide is available at http://earth.google.com/userguide/v5/.
DNR Garmin GPS Application is a free application developed by the Minnesota Department of Natural Resources and was built to provide users the ability to directly transfer data between Garmin GPS handheld receivers and various GIS software packages. Using this program a user can use point features (graphics or shapefile) and upload them to the GPS as Waypoints. Line and Polygon Graphics or shapes can be uploaded to the GPS as Track Logs or Routes. Conversely, Waypoints, Track Logs, and Routes collected using the GPS can be transferred directly to ArcView/ArcMap/Google Earth/Landview and saved as Graphics or Shapefiles. This program has a real-time tracking mode that allows users to follow their progress on the ground within an ArcView View Document, ArcMap Data Frame, or Landview Map. This real-time track log can be saved as either points or lines as a set of graphics or in a shapefile.
A list of free available software is at http://www.maps-gps-info.com/fgpfw.html.
GPS Conservation Applications
GPS is readily used in land surveys to note the precise coordinates of property boundaries. Because the precise measurement of these positions is one of the fundamental elements of land surveying, GPS has become a valuable tool as it provides a higher degree of accuracy over a large area than other methods . (GPS does not eliminate error completely since miscalculations may occur due to varying environmental and weather conditions, gravity (depending on your altitude) and of course human error.) The weather and other atmospheric conditions can have an adverse effect on the accuracy and dependability of the GPS. Anomalies like high static discharge during lightning storms, humidity, ionized air and barometric pressure can cause false information, data error or miscalculations. In many instances where GPS is used, these types of miscalculations will have little or no impact on the user’s desired outcomes; however, where accuracy is essential, such as in the case of land surveying, these types of miscalculations can be problematic.
Surveyors have traditionally relied on landmarks that can be destroyed or moved over time. GPS uses exact coordinates rather than relational landmarks. Therefore, using GPS as a land surveyor produces measurements that will be accurate no matter what happens to the surrounding land and/or landmarks.
Although GPS receivers allow for very precise measurements, there is still a degree of error involved. Many receivers will deploy a clustering approach in which each time a measurement is taken, a slightly different position is recorded, creating a cluster of points around the actual location. The more sophisticated the receiver is, the more accurate the data that is collected. For example, survey-grade receivers may produce a group of measurements clustered within just one centimeter of the actual location. Still, surveyors using GPS may have trouble surveying areas that are heavily wooded or contain large obstructions. As technology improves, the accuracy improves. GPS technology is rapidly advancing, providing valuable tools for land surveyors.1
Prioritization and Planning
Data collected through GPS technology can be very helpful to organizations in guiding conservation planning efforts and prioritizing protection efforts.
When a new conservation easement is established, GPS can be used to collect information about that property and help to create detailed maps as part of the baseline report.
GPS points can be taken at photo points in the field and then the photos can be uploaded as a point layer into the GIS database. Digital photos acquired during a GPS survey can provide a visual record of important features along the survey route. But for this record to be meaningful, each photo file should be linked to the geographic coordinates where it was acquired.
By using GIS technology, users can obtain critical information on sediments and separate soil series based on elevations and landforms. GPS technology can improve efficiency, quality and accuracy.
In addition, a wide variety of natural features on the ground can be mapped using GPS while out in the field including invasive species, soil erosion, fire disturbed areas, trees or plants of special concern, lakes, riparian areas, and streams. In addition, a wide variety of man-made features on the ground can be mapped using GPS while out in the field including buildings and other structures, roads, driveways, trails, fences, powerlines, levees, etc.
Locating and Relocation Features in the Field
It is also possible to upload GIS data onto your GPS unit and navigate to those locations while in the field. For example, if during the baseline mapping a patch of invasive plants was recorded, you could upload that information and relocate the area in subsequent years.
Organizations may create protocols and best management practices for collecting data within the organization. However, the two primary rules are consistency and accurate documentation. The user should always report in the metadata the source of the data and the expected precision of the data. When documented appropriately, data can be easily used with other data with minimal adjustments, and others using the data in the future will know exactly what format (projection and datum) was used to collect the data. If data is not appropriately documented, it is possible that it will be unusable in the future.
A 2004 article by Melissa K. Thompson states that:
”In case law that mentions evidence derived from GPS, frequently the courts refer to the technology in a matter of fact way, and its admissibility is not an issue separate from rights of privacy claims or search and seizure laws. Courts’ general acceptance of information based upon GPS may be a result of the technology’s many years in service and its relatively ubiquitous commercial nature. Land trusts and public agencies that hold conservation easements can rely on GPS technology with a fair degree of confidence in the usefulness of the information it produces and its admissibility in a court of law.” (Thompson, Melissa. “The Legal Efficacy of New Technologies in the Enforcement and Defense of Conservation Easements”. Exchange (Summer 2004): pp 15-19).
Users of GPS should take care to recognize that the courts' view of GPS data will likely vary depending on who collected the data, with what type of equipment and for what purpose. It is highly likely that a court will accept a property boundary determined by a licensed surveyor using GPS. Far less certain is how the same court might view a boundary between protection areas within a conservation easement that was recorded only on paper based on points gathered by a land trust staffperson's consumer-model GPS receiver. Organizations should discuss with their legal counsel the appropriate application of GPS and relative legal risks involved given the most recent developments in statutory and case law.