Assessment of GIS Data Collection Software for High Precision DGPS in Surveying and Engineering

Mapping Applications

Yousif Al-Ghamdi Surveying Services Division

Saudi Aramco, Dhahran Saudi Arabia

yousif.ghamdi.12@aramco.com

and

Mark Bowhay Surveying Services Division

Saudi Aramco, Dhahran Saudi Arabia

mark.bowhay@aramco.com ABSTRACT Surveying and engineering mapping applications require high precision spatial data collection techniques to achieve their required accuracy. However, for users of these applications to exploit the power of GIS technology they may have no choice but to sacrifice the accuracy they are after. The main reason behind this observation is that today’s GIS data collection software packages lack some of the fundamental requirements of such precision mapping, namely high accuracy horizontal and vertical datum transformation capabilities, and standard surveying and engineering field practice tools. Following an introduction to precision mapping applications and the revolution in high precision differential GPS (DGPS), this paper discusses a few incompatibility issues with three recognized GIS data collection software packages and the fundamental requirements of precision mapping. Solutions for bridging the gap between GIS technology and precision mapping using OmniSTAR High Precision (HP) DGPS will be briefly addressed in this paper. Key Words: GIS data collection, High Precision DGPS, datum transformation, precision mapping.

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1. INTRODUCTION Much of the background work that is expressed in this paper comes from research by the Land Survey Section of Surveying Services Division of Saudi Aramco into data collection for ArcGIS software. When we started we never anticipated that our precision mapping requirements would be so difficult to implement when using what is in effect industry standard software. With the introduction of RTK GPS in the 1990s our organization established a WGS-84 control network to coincide with the existing horizontal and vertical control networks in use at the time. This work was performed to create a consistent kingdom wide transformation method which would allow field crews to perform their work seamlessly without the need to select local parameters or to perform local control calibrations. These transformation methods are required because the Aramco surveying and mapping is carried out in the Ain Al Abd-1970 datum while GPS uses the WGS-84 datum To achieve this, the Surveying Services Division developed kingdom wide datum and geoid transformations in the form of grid-based models. These models proved to be easily applied to the Trimble Survey and GIS data collection software in use at the time. Ground based mapping and surveying traditionally used relative methods (ties to nearby ground control) to execute the survey. These techniques used by Total Station (angle and distance measurement) surveys apply equally to RTK GPS survey methods. These relative methods ensured that the collected data was compatible with the local survey control. RTK GPS, while providing the required accuracies for precision mapping, is in practice an expensive operation and it is our intention to reduce precision mapping costs by replacing RTK GPS with more cost-effective GPS technology. DGPS correction services have been available in the Arabian Gulf area for some time, broadcast from

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terrestrial radio stations. These services have in general not been suitable for precision mapping with quoted accuracies of 1-3 meters horizontally and less vertically. Networks of GPS control stations and user broadcast corrections would have provided the required precision mapping accuracies, but with station spacing at one to two hundred kilometer intervals across the kingdom, would have been too expensive to implement. The introduction of High Precision DGPS services based on satellite broadcast corrections and GIS based data collection and mapping software gave us opportunities to improve our data collection efficiency by reducing man power and equipment costs. The objective of the study behind this paper is to find methods and tools to enable the use of existing mapping and GIS software in the acquisition of precision mapping data as defined in this paper. We therefore started looking for a software application which would enable relatively unsophisticated operators to perform their duties in as seamless a process as possible. It seemed to us that it should not be necessary for users to have to reconfigure data collection software at every major change in location. This paper gives an introduction to surveying and engineering precision mapping applications performed by the Land Survey Section and outlines the major requirements of precision mapping. It also highlights the revolution of high precision DGPS and discusses a few incompatibility issues with three recognized GIS data collection software packages showing the gap between GIS technology and precision mapping in this regard. 2. PRECISION MAPPING APPLICATIONS Surveying and engineering applications generally demand mapping at a precision level higher than that which is commonly accepted in a GIS system. In order to realize this higher level of mapping, it is

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fundamental to understand the terms used to describe the quality of measurements, and consequently, the quality of maps. 2.1. Accuracy vs. Precision Several terms are used to describe the quality of measurements and information derived from them. The terms accuracy and precision, although widely misunderstood to mean the same, are terms most commonly used in surveying and engineering. The term accuracy is the nearness between measurements and their true value. The further a measurement is from its true value, the less accurate it is. As opposed to accuracy, the term precision is the degree in which several measurements provide similar results. In other words, it pertains to how repeatable measurements are. The grouping of rifle shots on a target is an example often used to demonstrate the difference between the two concepts. Figure 1 shows four different groupings that are possible to obtain. Based on the preceding definitions, both terms are clearly distinguishable in this example.

High Accuracy High Precision

LowLow

LowLow

Accuracy High Precision

High Accuracy Precision

Accuracy Precision

Figure 1: Rifle shot groupings. It must be noted that while the precision of any measurement is only affected by the measuring instrument, the accuracy of any measurement is the product of accuracies of the components of that

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measurement. For example, a GPS measurement is only as accurate as the sum of the instrument accuracy, the control accuracy, and the map projection and datum transformation accuracies. 2.2. Surveying and Engineering Mapping Applications In the context of our surveying and engineering operations, we see the applications of precision mapping in the following fields: 1. Cost effective data collection for the survey and design of oil and

gas field facilities, particularly pipelines and power lines. 2. Accurate location of new and proposed oil and gas wells. 3. Quality control and updating of photogrammetric mapping at all

scales, with accuracies approximately an order of magnitude better than the accuracies obtained from photogrammetry. (Precision mapping is particularly suited for this application.)

4. Horizontal and vertical control of photogrammetric mapping. 5. Project feasibility surveys and project proposal quantity surveys. 2.3. Defining Precision Mapping With regard to the applications outlined in section 2.2 above, we would define high precision mapping as follows:

Data collection with horizontal accuracies of better than 0.1 m 95% of the time and with vertical accuracies of better than 0.2 m 95% of the time.

3. THE REVOLUTION OF HIGH PRECISION DGPS Usage of the Global Positioning System (GPS) in surveying and mapping has been increasing dramatically with improved GPS methods and accuracies. Differential GPS (DGPS) has been one of the most used GPS methods in surveying and mapping, particularly for GIS data collection.

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3.1. DGPS Background Differential GPS (DGPS) is simply a GPS relative-positioning method that determines one location with respect to another location. In DGPS, GPS autonomous precision (10-30 m) is increased using differential corrections by comparing the readings of two receivers, one fixed at a known location, and the other roving. DGPS can provide the user post-processed or real-time, three-dimensional positions. See Figure2.

Figure 2: Differential GPS Positioning1

Real-time DGPS correction data can be obtained from worldwide terrestrial radiobeacons (see Figure 2) or from a variety of commercial wide-area augmentation systems that use geo-stationary satellites to send the corrections, such as Fugro’s OmniSTAR, covered in Section 3.2. When using this technique with the C/A or P-code, it is called differential code phase positioning, being distinct from differential carrier phase positioning, both explained hereafter.

1 Courtesy of Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder.

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3.1.1 DGPS Code Phase Positioning Also known as pseudorange differential, DGPS code phase positioning refers to relative positioning using the C/A or P code-phase observables providing meter level precisions. Although it has limited application to detailed engineering control surveying and topographic site plan mapping applications, DGPS code phase positioning is widely used for general reconnaissance surveys, navigation, preliminary stake out surveys, and hydrographic surveys. Terrestrial worldwide radiobeacons transmitting DGPS radio signals fall under this category. 3.1.2 DGPS Carrier Phase Positioning DGPS carrier phase positioning is used to obtain the highest precision from GPS and has direct application to most surveying and engineering mapping applications. This method measures a 3-D baseline vector between the base station (receiver occupying a known point) and a second receiver at another point (rover) resulting in a vector difference between the two points occupied. Real-Time Kinematic (RTK) is a DGPS method that falls under this category and can potentially provide centimeter level accuracy. However, drawbacks to RTK systems include the requirement of two sets of expensive GPS receivers, radio links, at least a 2-man and 2-vehicle crew, and the limitation of a ±10 km base-to-rover distance. In recent years, new dual frequency carrier phase based real-time DGPS services have emerged to provide decimeter level accuracy over wide areas. These services use geo-stationary satellite links to provide wide area coverage. The new positioning methods bridge the accuracy and coverage gap between meter level code based DGPS and expensive centimeter level RTK systems. Hence, the Land Survey Section saw the potential in utilizing such methods as the search for more efficient data collection methods continues.

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3.2. OmniSTAR DGPS OmniSTAR is typical of the commercial fee-for-service wide area DGPS system using geo-stationary satellite broadcast techniques to deliver accurate GPS correctors. Data from many widely spaced reference stations is used in a proprietary multi-site solution to achieve sub-meter positioning over most land areas worldwide. OmniSTAR is a proprietary system operated by the FUGRO group. The OmniSTAR concept is illustrated in Figure 3.

Figure 3: 1. GPS satellites. 2. Multiple OmniSTAR reference stations. 3. Send GPS corrections to 4. GPS monitor network control centers where data corrections are checked and repackaged for uplink to 5.

Geo-stationary L-band satellite. 6. The satellite broadcasts footprint on earth = OmniSTAR user area. 7. Correction data are received and

applied real-time.2

2 Courtesy of OmniSTAR USA, Inc.

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3.2.1 OmniSTAR HP The recently introduced OmniSTAR-HP (High Performance) solution has revolutionized the use of DGPS in precision mapping applications over entire continents without the need to deploy local base stations. It is a dual frequency GPS augmentation service that uses differential carrier phase positioning with multiple reference stations and eliminates modelled errors. This makes OmniSTAR HP capable of providing positional accuracy of better than 10 cm (95%) horizontally and better than 20 cm (95%) vertically up to 1000 km away from reference points. The OmniSTAR HP (1000 km) coverage map for Saudi Arabia is shown in Figure 4.

Figure 4: OmniSTAR HP service coverage map showing 1000 km

radii from the four available reference points in our region.3

3.2.2. Issues with OmniSTAR HP The OmniSTAR HP correction data are based on the International Terrestrial Reference Frame (ITRF), a high-accuracy version of WGS- 3 Courtesy of FUGRO.

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84, the datum used by GPS, that has been created in a number of time-based realizations since 1989 with the latest being ITRF-2000, and is most suitable for international high-accuracy applications. However, the problem with using a global coordinate system for precision mapping in a particular country or region is that continents are constantly in motion with respect to each other at rates of up to 12 cm per year. Based on a preliminary study performed by the Land Survey Section in early 2004, the Saudi Aramco WGS-84 datum is moving northeasterly at a rate of approximately 3 cm per year relative to ITRF-2000. The bottom line is that in order for a user to obtain the level of accuracy that the system is capable of, a high accuracy link (transformation) between ITRF-2000 and the user’s local datum is essential. In 2004, the Land Survey Section purchased two OmniSTAR 8300HP DGPS receivers, which use built-in Novatel GPS receivers, to augment their existing DGPS capabilities and to investigate the replacement of some existing RTK GPS systems. It was evident from preliminary field tests that the system should be capable of meeting the requirements of precision mapping as discussed hereafter. 4. REQUIREMENTS FOR PRECISION MAPPING 4.1. Geodetic Datum Transformation To realize the full potential of OmniSTAR HP over a large area, kingdom-wide as in our case, a number of precision mapping requirements need to be taken into consideration. Most importantly is the existence and implementation of high accuracy geodetic datum transformation; horizontally between the ITRF-2000 global datum used by OmniSTAR and Saudi Aramco’s version of Ain Al Abd-1970 horizontal datum, and vertically between ITRF-2000 ellipsoidal heights and the Saudi Aramco Vertical Datum (SAVD-1978) Mean Sea Level (MSL) elevations.

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The coordinates of a point will change depending on which datum the coordinates are referred to. To change a coordinate from one datum to another, a mathematical process known as transformation is used which requires a number of points with positions known in terms of both datums (common points). The accuracy of the transformation depends on the method chosen, and the accuracy, number, and distortion of the common points. Geodetic datum transformations are usually geocentric 3, 7, and 14 parameters or grid-based. Local transformation methods are outside the scope of this discussion as our focus is on Earth-centered coordinate systems only. 4.1.1. High Accuracy Transformation A grid-based transformation is considered the most accurate, consistent, and efficient transformation suitable for precision mapping over large areas such as Saudi Arabia. Surveying Services Division has developed its own horizontal grid-based model and vertical geoid grid-based model. 4.1.1.1. Horizontal Grid-based Transformation In late 1990s, Surveying Services Division developed and implemented a horizontal grid-based transformation model due to the increased dependence on GPS RTK systems, from Trimble, and the demand for a seamless transformation methodology between latitudes and longitudes of WGS-84 GPS datum and Ain Al Abd-1970 Saudi Aramco used datum. This model consists of NADCON files (.las and .los binary format) that contain coordinate shifts for latitudes and longitudes, respectively. A bi-linear interpolation process uses the model to easily, on-the-fly provide Ain Al Abd-1970 coordinates from WGS-84 GPS coordinates. This model is compatible with Trimble’s data collection software.

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4.1.1.2. Geoid Grid-based Transformation Similarly, a geoid grid-base transformation model has been utilized for the transformation of WGS-84 ellipsoidal heights to the Saudi Aramco Vertical Datum (SAVD-1978) Mean Sea Level (MSL) elevations. The model, a .bin format, is composed of undulation (N) values representing the separation between the ellipsoid (WGS-84) and the geoid (MSL) across the kingdom. The formula used to derive the MSL height is: H = h – N Where: (H) is Mean Sea Level height. (h) is ellipsoidal height. (N) is undulation value. A bi-quadratic interpolation process using the model is performed to provide on-the-fly MSL elevations. Again, this model is compatible with Trimble’s data collection software. 4.1.2. Medium Accuracy Transformation The relationship between two consistent, 3-D coordinate systems can be defined by a seven parameter transformation (three origin shifts, three rotations, and a scale change). If used over a large area, Saudi Arabia for instance, a 7-parameter transformation could be suitable for medium accuracy projects, of the order of 1 m. However, a 7-parameter transformation can be sufficient for small areas, a city for example, yet not be suitable for precision mapping over large areas. To make an allowance for differences in reference frames over time, a slightly more complex 14-parameter transformation (7-parameters plus their rates) can be used as a better long-term practical solution for datum transformation. A 14-parameter transformation allows users

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to map a 7-parameter transformation to any epoch of interest. For the same purpose, it can be applied together with a grid-based transformation. Because the differences between the current ITRF GPS datum and the Saudi Aramco WGS 84 datum are fairly uniform, a 7-parameter transformation is adequate to convert from the OmniSTAR HP position to the Saudi Aramco WGS-84 datum. It is planned to increase the number of observations between the two systems and to hopefully develop a 14-parameter transformation containing both vectors and velocities which should within the accuracies required for precision mapping give a time independent solution. 4.1.3. Low Accuracy Transformation A 3-parameter transformation, such as Molodensky’s transformation method, is considered a low accuracy transformation which uses geocentric X, Y, and Z translations only. This method of transformation is useful only for navigation purposes outside very limited areas and so is generally not applicable to precision mapping. It is often used in hand-held GPS receivers with the 3-parameters published by the United States National Imagery and Mapping Agency (NIMA). 4.2. Surveying and Engineering Practice Requirements 4.2.1. Hardware Requirements Field computers, as well as GPS units, must be rugged. That is, they must be capable of being used 8 to 10 hours a day in harsh conditions, be waterproof, sand proof and be tolerant of moderate physical abuse. They must also be light, possess good power reserves (capable of a days work on a single charge), and above all, be user friendly.

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The environment in Saudi Arabia is extremely demanding in terms of computer design, most notably in terms of ambient temperature and brightness. While a number of manufacturers are able to provide hardware capable of operating in temperatures in the high 40s, very few are able to provide battery operated computers whose displays are sunlight readable at midday in the Saudi Arabian summer. This is to a large part due to the lack of available large format reflective or transflective technology display screens. 4.2.2. Software Requirements The software used in precision mapping has to conform to a number of requirements listed here: 1. The software must be capable of working on the field computer

selected. 2. It must be compatible with the data formats of the GPS unit

selected. 3. It must be compatible with the data format and workflow of the

office based mapping software. 4. It must be capable of meeting the accuracies required. 5. It should be uncomplicated in field operations, but flexible enough

to meet the operational requirements. 6. It should be capable of customization in order to meet the above

criteria should they not be available ‘out of the box’. 5. ASSESSMENT OF SELECTED GIS DATA COLLECTION SOFTWARE The Land Survey Section conducted a broad review of the available high precision GIS data collection hardware and software tools, and while there were no preconditions, there was an emphasis placed on interoperability with the office based software (ArcGIS) and on field usable hardware.

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As the Land Survey Section was unable to locally source a suitable rugged Windows Tablet PC, the decision was made to concentrate on the PDA format hardware platform which is available in rugged packaging and which uses transflective screen technology. Almost all currently available software for this platform is based on the Pocket PC or Windows CE operating systems. From the available precision mapping and GIS data collection software, the following three packages were selected for evaluation. 5.1. ArcPad Originally a third party product, this package is now supported directly by ESRI. In spite of having many shortcomings in its present incarnation (ArcPad 6.0.3), ArcPad was the package which we currently believe to be most suitable to meet our requirements.

Figure 5: ArcPad interface.4

4 Courtesy of Environmental Systems Research Institute, Inc. (ESRI).

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As expected, ArcPad has the best integration with ArcGIS and the shortcomings can be overcome with the customization options, which are the most comprehensive of the three packages considered. The shortcomings however were serious. ArcPad did not support geoid grid modeling, datum grid transformations and, unexpectedly, did not have built-in support for GPS antenna offset. This more than anything else highlighted the difference in approach between Surveying and GIS software. The customization options available include custom toolbar and form creation, Visual Basic scripting, and most importantly from our perspective, support for custom GPS extensions. While no explicit support is offered for the OmniSTAR 8300HP DGPS receiver, the built-in NMEA GPS interface proved to be flexible enough to permit improved instrument initialization through ‘Seeding’ and the recording of an antenna offset, both implemented using custom forms and the VBS scripting. It was soon realized however that due to some bugs with the binary file access in VB Scripting, the only way to handle geoid modeling and datum grid transformations would be by the implementation of a custom GPS extension. The combination of the built-in OmniSTAR 8300HP 7-parameter transformation from the current ITRF frame to the Saudi Aramco WGS-84 datum, and the datum grid and geoid model transformation from the Aramco WGS-84 datum to the Ain Al Abd-1970 datum, together with implementation of antenna offset support in the extension, gave us Real Time Precision Mapping using the OmniSTAR HP service. Recent developments from ESRI with ArcPad 7, in beta testing at the time of writing, have confirmed our decision to proceed with ArcPad. Improvements to ArcPad in version 7 include support for datum grid transformations, support for antenna offsets, very much improved graphics, the ability to create transformation extensions as well as

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GPS extensions and an improved interface to the Desktop ArcGIS packages. It is still disappointing to us that geoid modelling is not supported directly by ESRI in their software. However, with the support for custom extensions and with, at last, explicit support for antenna offsets and user defined geoid separation values, steps are being taken to bridge the gap between Survey and GIS data collection software. 5.2. Imap Imap, from Point Inc. and Sokkia, has on the surface many of the features required by our application. This software is an OEM version of the Solo Field application from Tripod Data Systems (TDS) and any comments would apply to both packages. While support for NADCON datum grid modeling is available, it would appear that from our review of the software that this facility is hard coded into the application and could not be configured for our circumstances. Support for custom datums and transformations is limited internationally to three and seven parameter transformations. Explicit support for the OmniSTAR 8300HP DGPS receiver is available, as the Sokkia 6250 receiver, like the OmniSTAR 8300HP, is an OEM version of the Novatel Propack LB. This support extends to the ‘seeding’ function of the OmniSTAR service, in which the receiver initialization time can be reduced by enabling the position solution to converge on a known point. Like Pocket GIS mentioned below, this software depends on a PC based application to both manage configurations and also to transfer data between the field data collector and the office. Together with the inability to access our custom datum grid transformation, lack of geoid grid modeling and almost no local support, this software gave us no compelling reason for us to adopt it as our data collection package.

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5.3. PocketGIS One of the few packages with explicit support for the OmniSTAR service and the 8300HP receiver, this software also provides support for a two-stage transformation, allowing for a transformation from the current ITRF reference frame to the local WGS-84 datum and thence to a local user datum. This facility however is not required for the OmniSTAR/Novatel receiver as the receiver itself is able to handle a 14-parameter transformation. Good support is also available for elevation, however geoid modeling is not supported which would mean an additional stage in the data transfer process. Data is transferred to and from PocketGIS by means of an auxiliary translation program called PocketGIS Connection. A number of data formats are supported including among others, shape files. However this would in our case involve a second stage of conversion from the ArcGIS coverage files to shape files and finally into PocketGIS. This together with the lack of support, at the time of our evaluation, for datum grid models and the relative lack of user customization options reduced the appeal of this package for our application. It should also be noted that together with most other GIS data collection software, this program lacks a ‘calibration’ function. This function is almost always available for RTK GPS surveying field data collection software and would restrict the use of both this software and also the concept of using High Precision DGPS for many users.

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6. RESULTS AND CONCLUSIONS A number of conclusions can be drawn from our work on this paper as well as recent experiences. These are as follows: 1. The convergence between surveying and GIS has not yet been

bridged, although an increasing number of individuals and companies are becoming aware of the advantages of doing so.

2. Too much software is crippled by a lack of configurable options.

The software has the basic functionality but it is simply not configurable by the user. Whether this is a design shortcoming or a result of a design philosophy, is open to debate.

3. Parochial attitudes are often taken to software design. A typical,

and somewhat simplistic, example is that European software is often designed around an Easting/Northing display and input format while US originated software is built around a Northing/Easting layout.

4. Some manufacturers would appear to have decided not to support

the technologies discussed in this paper. On the surface this would appear to be a commercial decision, probably to avoid impacting the sales of more expensive products, as the individual components exist within their product lineups.

5. Proprietary solutions are causing problems for many users as too

much hardware and software is designed to be a one-stop-shop and does not work well, if at all, with components from other manufacturers.

6. Diluted support from international companies working through local

offices or agents who do not themselves appreciate the problems nor do they have the skills and knowledge required to address them.

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7. One of the most important decisions regarding the introduction of this technology is the selection of a hardware platform; the selected computer must be usable and reliable under field conditions.

8. The perceived lack of truly usable large display format field

computers is adversely affecting the introduction of mobile mapping technologies; the ¼ VGA screen format of the typical PDA is barely adequate for mapping purposes.

9. The suppliers of the Precision Mapping services would appear to

be adversely affected by the lack of readily usable software in many of their markets. This is most apparent in the less populated and developed parts of the world.

10. Finally, the shortcomings in any software product can be overcome

by the robust support by the authors and manufacturers of extensive third party and user customization.

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