PLOTTING SURVEYING DATA IN GOOGLE EARTH

Transcription

1 PLOTTING SURVEYING DATA IN GOOGLE EARTH D M STILLMAN Abstract Detail surveys measured with a total station use local coordinate systems. To make the data obtained from such surveys compatible with Google Earth, a virtual globe that uses global coordinate system WGS84, a geodetic transformation is required. The paper discusses the different coordinate systems used within Great Britain and those used by Google Earth. The paper goes on to describe the computer program developed to generate KML documents from surveying data, so that it can be visualised within Google Earth. It also describes accuracy tests carried out to assess the accuracy of the aerial imagery used within Google Earth as well as the accuracy of the Helmert transformation used by the program. Introduction Google Earth is a free computer program which uses a virtual globe to map the Earth using superimposed satellite and aerial imagery. Since Google Earth produces imagery which represents the whole world, it uses global GPS coordinate system WGS84. There are many tools which currently exist for viewing GPS data within Google Earth. However, prior to this project there was no way of easily converting data obtained from land surveying, using local coordinate system OSGB36, for visualisation within Google Earth. Doing so will allow the surveyor to view their data in relation to the imagery and terrain and will also enable the surveyor to quickly check that all necessary data has been captured. Background There are four main areas that need to be considered, they are land surveying, coordinate systems, conversions and geodetic transformations, and Google Earth. Land Surveying Total stations measure horizontal angles, vertical angles and distances from a single set up. Detail Surveys use total stations to measure all of the detail from a control framework required to produce a plan or map. Once a control framework is set up, the total station can be set up at a control station and detail can be measured by setting a prism on a detail point. Once all of the detail has been measured, the raw data file from the total station can be reformatted into the more readable and standard form of a field file. This can be done using software such as LISCAD (2009). The field file can then be edited to insert information about the control stations the detail points were measured from. This new file is called an edited field file, and can be used to find the coordinates of all of the detail points. Coordinate Systems The coordinate system traditionally used in Great Britain is OSGB36 (Ordnance Survey Great Britain 1936), a two-dimensional ellipsoidal coordinate system based on the Airy 1830 ellipsoid. The Airy 1830 ellipsoid is a simplified shape of the Earth chosen as it fits the Earth particularly well for the region. The Transverse Mercator Plotting Surveying Data in Google Earth 1

2 map projection is commonly used with OSGB36 to give coordinates in Eastings and Northings instead of latitude and longitude. Heights used in Great Britain are based on the Ordnance Datum Newlyn (ODN). This is a one-dimensional coordinate system. The heights are geoid heights heights relative to mean sea level, which is a tide-gauge in Newlyn. Google Earth uses GPS coordinate system WGS84 (World Geodetic System 1984) for latitude and longitude, and Geoid model EGM96 (Earth Gravitational Model 1996) for heights. WGS84 is a coordinate system based on the GRS80 (Geodetic Reference System 1980) ellipsoid, an ellipsoid which fits the whole Earth the best. Coordinate systems can be known as datums or Terrestrial Reference Systems (TRSs) when they have been fixed to the Earth by a datum definition. Terrestrial Reference Frames (TRFs) are set up to realise the datums so that they can be used in the real world. For example, OSGB36 traditionally used trig pillars on hill tops as points of known coordinates, enabling new points to be derived by measuring from the pillars. Conversions and Geodetic Transformations Coordinate conversions are different to transformations in that they do not involve a change of datum (Iliffe and Lott, 2008). Examples of conversions are going from ellipsoidal coordinates to Cartesian coordinates, or to map coordinates using a projection. The parameter values for the conversion are defined and as such they do not lose accuracy and are reversible to get the same answer. Transformations on the other hand are used to transform coordinates between different datum realisations and will be affected by the surveying imperfections of both coordinate reference systems. Two ellipsoidal datums can differ in position of the origin of coordinates, in the orientation of coordinate axes, and in the ellipsoid size and shape. The ellipsoid size and shape can be eliminated most simply by converting to three-dimensional Cartesian coordinates. A Helmert datum transformation can be applied to a TRF to rotate the Cartesian axes, translate the origin and alter the scale. The Helmert datum transformation does not take into account regional distortions in the TRFs, and as such using it to transform between OSGB36 and WGS84 can give errors of up to 4m (Ordnance Survey, 2007). Local transformation parameters can be used for a more accurate Helmert transformation. The Ordnance Survey has developed a more complicated transformation known as OSTN02 which takes into account the variable localised distortion. The OSTN02 transformation consists of grid translation vectors which cover the country at a 1km resolution. Bi-linear interpolation is used on the grid of translation vectors to calculate a shift corresponding to the local distortion. Google Earth Google Earth (2009) is a virtual globe, map and geographic information program. It is a freely available program that superimposes imagery obtained from satellite and aerial photographs onto a 3D model of the world. The user s geographic data can be represented easily on Google Earth through the use of Keyhole Markup Language (KML) documents. These documents can be used to show points, paths, polygons and ground overlays. Plotting Surveying Data in Google Earth 2

3 The vertical aerial photographs used in Google Earth have been georeferenced to align with the coordinate system. The process of georeferencing involves identifying ground control points in the image for which accurate coordinates are available. A transformation is then calculated by computer software which processes the image so that it aligns to the ground coordinate system (Wolf and Dewitt, 2000). Mosaics are used to stitch many aerial photographs together. Controlled mosaics use rectified photos so that all of the photos are vertical and at the same scale. In mosaic assembly, image positions of common features in adjacent photos are matched as closely as possible. A plot of control points is used to match and constrain positions, similar to the technique used in georeferencing. Uncontrolled mosaics simply match the image details of adjacent photos without using the ground control, which is quicker but less accurate in terms of the coordinate reference system. Semicontrolled mosaics have either no ground control or use photos that have not been rectified. Computer Programming The computer program was developed using Java, a modern object-oriented programming language. Java is portable, meaning programs developed with the Java programming language can be run on anything that supports the Java platform. Java is supported by all major PC operating systems, as well as many web browsers, mobile internet devices and mobile phones (Flanagan, 2005). The open-source integrated programming environment Eclipse (2009) was used for the development of the program. The program, named Google Earth Plotter, was chosen to use an approximate Helmert datum transformation. The steps required to change a set of land surveying coordinates (OSGB36 Easting and Northing and ODN height) to Google Earth coordinates (WGS84 latitude and longitude and EGM96 height) are as follows: (a) (b) (c) (d) (e) Convert grid Eastings and Northings to ellipsoidal latitude and longitude using map projection formulae; Convert ellipsoidal latitude, longitude and height to 3D Cartesian coordinates; Use the Helmert datum transformation to transform between OSGB36 and WGS84 TRFs; Convert 3D Cartesian coordinates back to ellipsoidal latitude, longitude and height; Discard WGS84 ellipsoid height. Use approximate Geoid height found by adding the EGM96-ODN height difference (0.8m) to ODN height. The computer program has three main functions. The first is to plot a single point, entered directly into the user interface by the user. The second function plots multiple points from a file, and the third plots an edited field file. All of the functions use the input data to create a KML file which includes the new WGS84 coordinates and formatting details. Google Earth uses the KML files to superimpose the data on top of its aerial imagery. Figure 1 shows the interface of the program for plotting a point and for plotting field data. Figure 2 shows field data plotted on Google Earth. Plotting Surveying Data in Google Earth 3

4 Figure 1. The Point and Field Data tabs of Google Earth Plotter Figure 2. KML representing field data collected during the surveying field course at Loughbrough University Plotting Surveying Data in Google Earth 4

5 To plot field data, the computer program has to convert detail points into coordinates using the horizontal angles, vertical angles and slope distances in the edited field file. Then the coordinates are transformed using the steps above. The control stations are plotted as triangles, with a thick white path joining the stations of the control framework (Figure 2). The control stations are coloured differently. The detail points are plotted as circles, with their colour corresponding to the control station they were measured from. The point IDs can also be shown as labels for the points. The user can specify the visibility of the labels with the program. They can be shown always, only when highlighted by the cursor, or not at all. The user also has the option to join consecutive points. This creates paths between points which have consecutive point IDs. The paths are coloured according to the control station used to measure the points. The paths are useful to view the order in which the points were measured. This can be useful to see what the points represent. For example, when points are measured along the side of a road, the path will represent the edge of the road. This gives the KML a more map-like quality. The user interface (Figure 1) also contains text boxes for the inputs of positional corrections. These corrections are used to align the KML with the Google Earth imagery. The corrections, in the form of East, North and vertical corrections, are simply added to the OSGB36 coordinates before the transformation is calculated. The user can find out the corrections required to align the KML with the imagery by measuring uncorrected KML with the Google Earth ruler tool. Accuracy Tests The accuracy tests made use of the Ordnance Survey s published coordinates for the passive stations of the National GPS Network. The coordinates are given in OSGB36 and WGS84. The stations used were pillars, as they can be located in the Google Earth imagery. The computer program was used to generate KML from the OSGB36 coordinates. The true WGS84 coordinates and the coordinates of the stations in the Google Earth imagery were added to the KML. This allowed the three different coordinate types to be compared. The Google Earth ruler tool was used to find the distance between the true (published) coordinates of the stations and the coordinates from the Helmert transformation and Google Earth imagery. The results of the accuracy tests are shown in Table 1. The averages are based on a sample of 17 passive stations. Two of the 17 stations were unable to be located in the imagery due to poor resolution. Distance Between Published and Imagery (m) Published and Helmert (m) Imagery and Helmert (m) Average Standard Deviation Table 1. Results of accuracy tests Plotting Surveying Data in Google Earth 5

6 Conclusions The program can quickly and easily plot survey data on Google Earth. The positional corrections used are only approximate as they do not take into account a rotation or scale error. But they have been highly effective in aligning the KML with the imagery. The accuracy tests show that the Helmert transformation used by the program is on average 1.3m horizontally from the position in the imagery. The true published values of the passive stations were even further from the imagery. This is an error of position in the Google Earth imagery. The accuracy of the aerial imagery depends on how well-aligned it is to the coordinate system used by Google s model of the Earth. The alignment accuracy depends upon how much time and effort has gone into georeferencing the vertical photographs, and the preparation of the digital mosaics. The Helmert transformation was found to be 2.1m from the published values. The use of OSTN02 would eliminate this error, however, it can be seen that this will not help the KML align with the imagery. The program can be used by a surveyor to create a KML document for data that has been measured in the field. This can be used as a quick check to verify the captured data, using the imagery to identify any missed areas. The surveyor can also forward the KML by to show the client the work that has been undertaken. Recommendations for Use The edited field file needs to be formatted in a specific way for best results. There must not be any blank lines, and IDs of detail points should be numerical only. This allows the program to plot lines between consecutive detail points. Download The program can be downloaded at: References Eclipse, About the Eclipse Foundation. < [accessed 23/04/2009]. Flanagan, D., Java in a Nutshell. 5th ed., Sebastopol: O'Reilly. Google Earth, Google Earth. < [accessed 04/05/09]. Liscad, LISCAD Surveying & Engineering Software. < [accessed 25/04/2009] Ordnance Survey, A guide to coordinate systems in Great Britain, Southampton: Ordnance Survey of Great Britain. Available at: Wolf, P.R. and Dewitt, B.A., Elements of Photogrammetry: with Applications in GIS. 3rd ed., Boston: McGraw-Hill. Plotting Surveying Data in Google Earth 6