Old Carson Methodology
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8181512.doc Created on 23/02/2008 18:11:00
Field Survey Operations and Procedures Field survey operations should be performed using the manufacturers recommended receiver settings and observation times. Operations under adverse conditions, such as in canopied areas, near tall buildings, or under unknown atmospheric conditions may require longer observation times than specified by the methodology described. Adjustable height antenna on a Trimble backpack or Laser Rangefinder staff will be used for all rover GPS observations. The elevation of an adjustable height antenna backpack\staff should be regularly checked to make sure height of the antenna is correct. All equipment should be periodically calibrated to ensure equipment accuracy. The following are recommended procedures when using GPS measurement techniques for inventorying city assets within the City of Carson. Mission Planning Mission planning shall be conducted before surveys to determine optimum conditions of daily acquisition unless otherwise instructed. This procedure enables the survey crew with appropriate information as to which type of capture and processing best suits the daily acquisition of point features. There are a number of methods for capturing and processing GPS data including: autonomous, realtime correction, and post-processing. Each of these methods will produce different accuracies. It is important for GPS users to be aware of what method they are utilizing and make sure any changes to the collection design is properly documented in the field journals and post-processing metadata. PDOP SNR Velocity Records GPS Dataloggers will be configured configuration settings for collecting City Asset Features unless otherwise stated. Positions acquired using GPS receivers are determined based on a measurement of time and distance. Receivers calculate the time and distance from a minimum of three satellite vehicles to acquire a horizontal position and four satellite vehicle to acquire a 3D (horizontal and vertical) position. There are sources of error that should be taken into consideration when collecting GPS positions. Atmospheric delay occurs when GPS signals are interrupted by the earth’s ionosphere and troposphere. As the signal is disrupted, the time it takes to reach the earth is altered, and can contribute approximately 1 meter of error. Since GPS positions are determined by time and distance, this alteration in time has an adverse effect on the accuracy of GPS fixed positions collected. The heat of the day is when atmospheric delay is greatest. Satellite vehicle signals low on the horizon also experience higher delays because the signal travels through more atmosphere.
Page 1
8181512.doc Created on 23/02/2008 18:11:00
Dilution of Precision (DOP) is the combination of error factors caused by poor satellite vehicle geometry that can alter position and time solutions. The receiver is using trilateration to determine its fixed position. If the satellite vehicles are not properly spaced the DOP values are higher and the horizontal position is degraded. Setting the receiver to accept positions when the PDOP is 6.0 or less (lower is better) will control this source of error by not allowing data with high DOPs to be saved by the receiver. Errors incurred by PDOP’s higher than 6.0 can cause approximately 1 meter of inaccuracy.
Fast Positioning Fast positioning will be used for observing City Asset Features unless otherwise stated. This method utilizes code-phase acquisition at shorter occupation times (i.e. 25 seconds – 90 seconds). The survey crew will use a combination of radial baseline and network techniques to correct for errors in autonomous position location. The radial baseline will be determined from City of Carson base station data while the network technique utilizes the Coast Guard Real-Time Correction (RTC) Radio Beacon from local CORS. An uncertainty of whether RTC from Coast Guard Radio Beacon requires least squares adjustment (more than 50 intervals) or use of processing software capable of producing a weighted mean average of the observations may affect positional accuracy. This uncertainty is under investigation and should be considered acceptable error if points are within 1 meter of known locations. Survey Transects All survey transects will follow a unidirectional circuit traverse survey for the collection of City Asset features if facing perpendicular to the transect line unless otherwise stated. This method of acquiring City Assets minimizes overlapping acquisitions in conjunction with adjacent points. If an objects location cannot be resolved using this method, the point will be collected and a comment should be inserted into the data dictionary for later update. In addition, the field handler should note the collection location by either sketching a quick annotated layout and\or noting the feature number in the field diary. A unidirectional traverse survey based on adjacent points will be acquired based on the following criteria: Single Family Residential (secondary roads)–Transects will be conducted either along edge of pavement or from centerline locations if conditions allow.
Page 2
8181512.doc Created on 23/02/2008 18:11:00
Commercial \ Industrial (primary\arterial roads)—Transects moving along the edge of pavement from South-to-North & West-to-East shall only capture features along pavement and inside the centerline (excluding median features). Conversely, transects moving along the edge of pavement from North-to-South and\or East-to-West shall only capture features along pavement and inside the centerline (including median features). Government (Misc. roads)—Traversing within government-owned\government-operated areas or along miscellaneous roads are considered special collection. As such, no a priori decision rules regarding collection apply so long as description of transects method is recorded in the field diary at time of collection. The city of Carson has established zones of acquisition to monitor progress of the City Assets Project. If for any reason the applied transect methodology conflicts with the areal bounds of these established zones, adjust the method of acquisition and log changes in your field notes. Reducing Errors A method for reducing multipath errors will be employed for observing City Asset features unless otherwise instructed. Multipath error occurs when a satellite vehicles signal is reflected off an object before reaching the receiver, thus causing a time delay. This can lead to an autonomous position with several meters of error inclusive. Buildings, trees, mountains, signs, etc, are potential obstructions to good signal reception producing multipath errors. Generally, errors are not easily recognized in the field and are best determined in-house when compared with a digital basemap in a Geographic Information System (GIS). Obstruction errors occur when the GPS receiver loses a lock from a satellite vehicle from which it is receiving code. Receivers and base stations both require a clear view of the sky. A clear view of the sky is determined by 75º of the field of view from 90º at zenith. In typical urban environments, multipath errors may account for up to 12 to 100 meters of horizontal error for a known location. As such, point survey sites should be assessed for two conditions: Non-Canopy\ No-Tall Buildings: For point features five positions of data is sufficient. Canopied\Tall Buildings: For point features 10 positions of data is recommended for data logged with real time beacon corrections. Otherwise, the point shall be collected with the laser rangefinder from an unobstructed location.
Page 3
8181512.doc Created on 23/02/2008 18:11:00
Non-Canopy\Canopy conditions are evaluated on-site by estimating if a fixed radial position is obstructed vertically more than 50% at 15º above horizon (>75º) and horizontally less than 75% (<250º \ 360º) unobstructed at 15º above horizon. To determine unobstructed horizontal conditions, a visual evaluation of the site may prove adequate to determine whether you exceed horizontal obstruction parameters. In this case, it is first necessary to estimate average height of structures near the point and estimate distance to those structures in four cardinal directions. For optimum survey conditions in a residential area, we assume there is a 90º Horizontal Field of View (HFOV) from the center of the street (lets say 60 feet to the edge of a tall structure). Though the horizon mask accounts for 15º in all directions leaving an omni directional 150º HFOV, the angle subtended from a single story dwelling is ~33º from the center of the road (114º of sky remain unobstructed). The combined radial horizontal distance can be less than or equal to 132º and should not exceed a combined 250º field of view in four cardinal directions. Below is an example of angles calculated at a location 10 feet from a given horizontal obstruction (20 ft side of a house). N 63º 10ft\20ft S 22º 50ft\20ft W 27º 40ft\20ft E 27º 40ft\20ft Caution: Operations under a canopy using RTC methods are not recommended. However, this method is acceptable if it results in a solution which meets survey standards expressed by the City. For City Assets in a canopied environment with marginal sky visibility, Fast GPS methods may not provide a satisfactory point solution and must be acquired from a laser rangefinder. The surveyor must make an informed decision when choosing the appropriate methodology to be used in any given project area. Datalogger Accuracy To determine if points collected during the project falls within established accuracy standards unless otherwise instructed. GPS receivers should be tested on a regular basis to ensure receivers are in proper working order. A zero baseline test will provide a degree of equipment reliability. Acquiring a fixed position on a known point on the earth’s surface, post-processing the data and comparing it to the known point is one way to determine a receiver’s working condition. A tabulation of Known Points Revisited (KPR) will be collected for estimating Circular Error (CE) and will be recorded in the field diary provided. A number of control points within the city of Carson can be utilized for checking Datalogger accuracy. For ease of use, however, two surveyed features within walking distance of city hall can be used for purposes of checking Datalogger accuracy so long as the features remains unchanged and their reference is indicated in the field journal throughout the project.
95% Confidence Circle
Page 4
8181512.doc Created on 23/02/2008 18:11:00
The 95% confidence circle representing a local accuracy can be derived from the major and minor semi-axes of the standard relative ellipse between two selected points. The 95% confidence circle is closely approximated from the major (a) and minor (b) semi-axis parameters of the standard ellipse and a set of coefficients. For circular error ellipses, the circle coincides with the ellipse. For elongated error ellipses, the radius of the circle will be slightly shorter than the major semi-axis of the ellipse. The radius r of the 95% confidence circle is approximated by: r = Kp a Where Kp = 1.960790 + 0.004071 C + 0.114276 C 2 + 0.371625 C 3 , C = b/a. Note that the coefficients in the above expression are specific to the 95% confidence level, such that when the major semi-axis of the standard ellipse is multiplied by the value of Kp , the radius of the 95% confidence circle is obtained directly, and no further conversion is required.
Page 5
Field Survey Operations and Procedures Field survey operations should be performed using the manufacturers recommended receiver settings and observation times. Operations under adverse conditions, such as in canopied areas, near tall buildings, or under unknown atmospheric conditions may require longer observation times than specified by the methodology described. Adjustable height antenna on a Trimble backpack or Laser Rangefinder staff will be used for all rover GPS observations. The elevation of an adjustable height antenna backpack\staff should be regularly checked to make sure height of the antenna is correct. All equipment should be periodically calibrated to ensure equipment accuracy. The following are recommended procedures when using GPS measurement techniques for inventorying city assets within the City of Carson. Mission Planning Mission planning shall be conducted before surveys to determine optimum conditions of daily acquisition unless otherwise instructed. This procedure enables the survey crew with appropriate information as to which type of capture and processing best suits the daily acquisition of point features. There are a number of methods for capturing and processing GPS data including: autonomous, realtime correction, and post-processing. Each of these methods will produce different accuracies. It is important for GPS users to be aware of what method they are utilizing and make sure any changes to the collection design is properly documented in the field journals and post-processing metadata. PDOP SNR Velocity Records GPS Dataloggers will be configured configuration settings for collecting City Asset Features unless otherwise stated. Positions acquired using GPS receivers are determined based on a measurement of time and distance. Receivers calculate the time and distance from a minimum of three satellite vehicles to acquire a horizontal position and four satellite vehicle to acquire a 3D (horizontal and vertical) position. There are sources of error that should be taken into consideration when collecting GPS positions. Atmospheric delay occurs when GPS signals are interrupted by the earth’s ionosphere and troposphere. As the signal is disrupted, the time it takes to reach the earth is altered, and can contribute approximately 1 meter of error. Since GPS positions are determined by time and distance, this alteration in time has an adverse effect on the accuracy of GPS fixed positions collected. The heat of the day is when atmospheric delay is greatest. Satellite vehicle signals low on the horizon also experience higher delays because the signal travels through more atmosphere.
Page 1
8181512.doc Created on 23/02/2008 18:11:00
Dilution of Precision (DOP) is the combination of error factors caused by poor satellite vehicle geometry that can alter position and time solutions. The receiver is using trilateration to determine its fixed position. If the satellite vehicles are not properly spaced the DOP values are higher and the horizontal position is degraded. Setting the receiver to accept positions when the PDOP is 6.0 or less (lower is better) will control this source of error by not allowing data with high DOPs to be saved by the receiver. Errors incurred by PDOP’s higher than 6.0 can cause approximately 1 meter of inaccuracy.
Fast Positioning Fast positioning will be used for observing City Asset Features unless otherwise stated. This method utilizes code-phase acquisition at shorter occupation times (i.e. 25 seconds – 90 seconds). The survey crew will use a combination of radial baseline and network techniques to correct for errors in autonomous position location. The radial baseline will be determined from City of Carson base station data while the network technique utilizes the Coast Guard Real-Time Correction (RTC) Radio Beacon from local CORS. An uncertainty of whether RTC from Coast Guard Radio Beacon requires least squares adjustment (more than 50 intervals) or use of processing software capable of producing a weighted mean average of the observations may affect positional accuracy. This uncertainty is under investigation and should be considered acceptable error if points are within 1 meter of known locations. Survey Transects All survey transects will follow a unidirectional circuit traverse survey for the collection of City Asset features if facing perpendicular to the transect line unless otherwise stated. This method of acquiring City Assets minimizes overlapping acquisitions in conjunction with adjacent points. If an objects location cannot be resolved using this method, the point will be collected and a comment should be inserted into the data dictionary for later update. In addition, the field handler should note the collection location by either sketching a quick annotated layout and\or noting the feature number in the field diary. A unidirectional traverse survey based on adjacent points will be acquired based on the following criteria: Single Family Residential (secondary roads)–Transects will be conducted either along edge of pavement or from centerline locations if conditions allow.
Page 2
8181512.doc Created on 23/02/2008 18:11:00
Commercial \ Industrial (primary\arterial roads)—Transects moving along the edge of pavement from South-to-North & West-to-East shall only capture features along pavement and inside the centerline (excluding median features). Conversely, transects moving along the edge of pavement from North-to-South and\or East-to-West shall only capture features along pavement and inside the centerline (including median features). Government (Misc. roads)—Traversing within government-owned\government-operated areas or along miscellaneous roads are considered special collection. As such, no a priori decision rules regarding collection apply so long as description of transects method is recorded in the field diary at time of collection. The city of Carson has established zones of acquisition to monitor progress of the City Assets Project. If for any reason the applied transect methodology conflicts with the areal bounds of these established zones, adjust the method of acquisition and log changes in your field notes. Reducing Errors A method for reducing multipath errors will be employed for observing City Asset features unless otherwise instructed. Multipath error occurs when a satellite vehicles signal is reflected off an object before reaching the receiver, thus causing a time delay. This can lead to an autonomous position with several meters of error inclusive. Buildings, trees, mountains, signs, etc, are potential obstructions to good signal reception producing multipath errors. Generally, errors are not easily recognized in the field and are best determined in-house when compared with a digital basemap in a Geographic Information System (GIS). Obstruction errors occur when the GPS receiver loses a lock from a satellite vehicle from which it is receiving code. Receivers and base stations both require a clear view of the sky. A clear view of the sky is determined by 75º of the field of view from 90º at zenith. In typical urban environments, multipath errors may account for up to 12 to 100 meters of horizontal error for a known location. As such, point survey sites should be assessed for two conditions: Non-Canopy\ No-Tall Buildings: For point features five positions of data is sufficient. Canopied\Tall Buildings: For point features 10 positions of data is recommended for data logged with real time beacon corrections. Otherwise, the point shall be collected with the laser rangefinder from an unobstructed location.
Page 3
8181512.doc Created on 23/02/2008 18:11:00
Non-Canopy\Canopy conditions are evaluated on-site by estimating if a fixed radial position is obstructed vertically more than 50% at 15º above horizon (>75º) and horizontally less than 75% (<250º \ 360º) unobstructed at 15º above horizon. To determine unobstructed horizontal conditions, a visual evaluation of the site may prove adequate to determine whether you exceed horizontal obstruction parameters. In this case, it is first necessary to estimate average height of structures near the point and estimate distance to those structures in four cardinal directions. For optimum survey conditions in a residential area, we assume there is a 90º Horizontal Field of View (HFOV) from the center of the street (lets say 60 feet to the edge of a tall structure). Though the horizon mask accounts for 15º in all directions leaving an omni directional 150º HFOV, the angle subtended from a single story dwelling is ~33º from the center of the road (114º of sky remain unobstructed). The combined radial horizontal distance can be less than or equal to 132º and should not exceed a combined 250º field of view in four cardinal directions. Below is an example of angles calculated at a location 10 feet from a given horizontal obstruction (20 ft side of a house). N 63º 10ft\20ft S 22º 50ft\20ft W 27º 40ft\20ft E 27º 40ft\20ft Caution: Operations under a canopy using RTC methods are not recommended. However, this method is acceptable if it results in a solution which meets survey standards expressed by the City. For City Assets in a canopied environment with marginal sky visibility, Fast GPS methods may not provide a satisfactory point solution and must be acquired from a laser rangefinder. The surveyor must make an informed decision when choosing the appropriate methodology to be used in any given project area. Datalogger Accuracy To determine if points collected during the project falls within established accuracy standards unless otherwise instructed. GPS receivers should be tested on a regular basis to ensure receivers are in proper working order. A zero baseline test will provide a degree of equipment reliability. Acquiring a fixed position on a known point on the earth’s surface, post-processing the data and comparing it to the known point is one way to determine a receiver’s working condition. A tabulation of Known Points Revisited (KPR) will be collected for estimating Circular Error (CE) and will be recorded in the field diary provided. A number of control points within the city of Carson can be utilized for checking Datalogger accuracy. For ease of use, however, two surveyed features within walking distance of city hall can be used for purposes of checking Datalogger accuracy so long as the features remains unchanged and their reference is indicated in the field journal throughout the project.
95% Confidence Circle
Page 4
8181512.doc Created on 23/02/2008 18:11:00
The 95% confidence circle representing a local accuracy can be derived from the major and minor semi-axes of the standard relative ellipse between two selected points. The 95% confidence circle is closely approximated from the major (a) and minor (b) semi-axis parameters of the standard ellipse and a set of coefficients. For circular error ellipses, the circle coincides with the ellipse. For elongated error ellipses, the radius of the circle will be slightly shorter than the major semi-axis of the ellipse. The radius r of the 95% confidence circle is approximated by: r = Kp a Where Kp = 1.960790 + 0.004071 C + 0.114276 C 2 + 0.371625 C 3 , C = b/a. Note that the coefficients in the above expression are specific to the 95% confidence level, such that when the major semi-axis of the standard ellipse is multiplied by the value of Kp , the radius of the 95% confidence circle is obtained directly, and no further conversion is required.
Page 5
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