GPS for GIS Data Collection - 101 Speaker: Eric Gakstatter, Editor of GPS World s Survey Scene Newsletter & Geospatial Solutions Guest Commentator: Craig Greenwald, Technical Director, GeoMobile Innovations, Inc.
Background (1) - Definitions GPS Global Positioning System -A constellation of ~30 satellites orbiting the Earth. -A GPS receiver uses measurements from several satellites to determine its position on the Earth. -GPS signals free to use by anyone anywhere in the world.
Background (2) - Definitions GIS Geographic Information System -Computer hardware and software used to store, manipulate, map and analyze geographic data. -Features (points, lines, polygons) are stored in a particular coordinate system (UTM, SPC, Latitude/longitude, etc.) with each having a unique coordinate on the Earth. -A table of descriptive information, called attributes, may be attached to each geographic location. -Data for a GIS come from a variety of sources.
Background (3) - Definitions GPS + GIS GPS is one tool that is used to acquire data for GIS. -It can be a handheld (or backpack) GPS system used by a person on foot to record detailed location and descriptive information on the ground. -It can be a digital camera with GPS capability. -It can be a GPS receiver continuously logging data as you re driving a rural road and recording a tracklog.
What Exactly is GPS used for in a GIS? -To record accurate position data on the ground (vs. remote sensing or other source of data). -To record detailed attribute information about the features you are recording. Example: Feature Name: Utility Pole Attributes: 60 ft high, AT&T, fair condition, Treated in 2008 Coordinate: 1783797.9, 13664696.6 (State Plane NAD83)
What equipment/software is needed? -Equipment Required: GPS receiver, data recording device. -Software Required: Data collection software. -There are many, many different configurations of GPS receivers, data recording devices and software, but primarily two distinct configurations exist: 1. All-in-one handhelds 2. Separate PDA and GPS receiver -The details of your project such as accuracy required and type of data to be collected will determine which configuration is best for you.
Consider your requirements
All-in-one vs. Stand alone -All-in-one = Simplicity -Stand-alone = Flexibility or or + or or
Software Considerations? -Support for form design and field data entry rules. -Coordinate system and datum support. -Compatibility between field device and desktop software. -Real-time syncing and multi-user editing in the field. -Integration with devices (laser, camera, barcode, probes). -Capturing GPS quality metadata. -Advanced features such as nested points, offsets, point averaging and all feature capture modes (point, line, polygons).
Field Software Choices
Equipment/software selection example -Utility pole mapping project: Budget = $5,000 -Required accuracy = 1 meter (horizontal -Data collection = point data with attributes. -Data collection environment = moderately clear sky. -The basic configuration would include a GPS receiver, data collector, data collection software, training, accessories. Optional: laser rangefinder.
GIS data = Point, Line, Polygon -Point = a single coordinate representing a single feature. Example - utility pole. -Line = a series of coordinates joined together to form a line. Example - Centerline of a road. -Polygon = a closed line that ends at the same point it started. Example Wetland area.
Point Averaging -By default, GPS receivers compute a position once per second. -For recording point features, users typically average data for as little as 3 seconds and for as long as 3 minutes. -The value of averaging can be deceptive. Better to average 5 seconds of stable, good quality data than 3 minutes of unstable, lower quality data.
Static vs. Dynamic -Line and polygon features can be collected using static or dynamic technique. -Dynamic (continuous) means line/polygon data will be stored once per second after your tap the STORE button. This technique assumes you are continuously moving. -Static data collection is like traditional traversing. Each vertices along the line/polygon is stored when you tap the STORE button. You must stop at each vertices along the line/polygon.
Offset Points -Sometimes it s not efficient or necessary to walk/drive up to each point and record it with GPS. -Some data collection software allows you to enter a distance and bearing from where you are located. -You either estimate the distance and bearing manually or use a handheld laser rangefinder to automatically record the distance and bearing.
Photo-geotagging -Take existing photos in the field for verification or field ident. -Shoot new photos as attributes or georeferenced.. -Associate photos taken in the field with corresponding features in the office. -Minimize file shuffle. -Minimize non-meaningful filenames. -EXIF extension to JPG to store metadata. Supports GPS and range data.
GPS How Accurate is it? Depends on: -Receiver quality -Type of correction method (of any) -Environment the GPS receiver will be used in. Generally speaking, the more accurate the GPS receiver, the more expensive it is.
GPS How Accurate is it? It depends -Receiver quality, type of correction method (of any) -Environment the GPS receiver will be used in. -GPS receivers can be classified by accuracy (horizontal): -cm-level -decimeter-level -sub-meter -1-3 meters -3-5 meters -5+ meters -Generally speaking: better accuracy = more expensive. -Vertical accuracy is at least 1.5 times worse than horizontal and even unusable on lower-end receivers.
GPS Differential Correction -Differential Correction improves the accuracy of GPS data and is necessary if you require for accuracy below 5 meters. -Real-time correction you receive corrections in the field wirelessly. -Post-processing you download reference GPS data after your field work and correct your data. Requires software. -Example methods of real-time correction services: SBAS (WAAS/EGNOS/MSAS), DGPS beacon, RTK Networks, OmniSTAR, Starfire. Some are free of charge, some are not.
GPS Data Quality -GPS signals are affected by obstructions (trees, buildings, terrain) that can degrade accuracy. -The more GPS satellites your receiver is tracking, the better. -The more spread-out (geometrically) the GPS satellites are in the sky, the better your positions will be. -The quality of the geometry of the satellites your receiver is tracking is indicated by the PDOP value. -Precision Dilution of Precision (PDOP) is displayed by all GPS receivers. The lower the PDOP, the better. A PDOP value of 1-3 is ideal. 3-5 is moderate. 5+ is poor.
GPS Gotchas -Unrealistic GPS accuracy expectations. -Unrealistic GPS tracking expectations. -Difficulty in accurately matching GPS-collected data to base map (datum transformation). -Unexpected hardware and/or software maintenance and training costs. -Not trying the system in your environment before purchasing. -Lack of metadata in GPS-collected data. -Lack of responsive or capable technical support.
QUESTIONS? Eric Gakstatter egakstatter@gpsworld.com Craig Greenwald craig@geomobileinnovations.com Subscribe to Survey Scene Newsletter at www.gpsworld.com/newsletters Subscribe to Geospatial Solutions Newsletter at www.geospatial-solutions.com/newsletters