Geomatics

“Geomatics s a relatively new term that is now commonly applied to encompass the areas formerly identified as surveying.  The principal reason for making the name change is that the manner and scope of practice in surveying have changed dramatically in recent years.  This has occurred in part because of recent technological developments that have provided surveyors with new tools for measuring and/or collecting information, for computing, and for displaying and disseminating information.  Today the modern surveyor’s arsenal of tools for measuring and collecting environmental information include electronic instruments for automatically measuring distances and angles, satellite surveying systems for quickly obtaining precise positions of widely spaced points, and modern aerial imaging and associated processing systems for quickly mapping and collecting other forms of data about the earth upon which we live.” (Wolf and Ghilani, Elementary Surveying, An Introduction to Geomatics, 2002)

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Geomatics

237 aes survey d237 aes-jfs d237 aeswriting dPrevious experience had shown that metric spatial data were necessary to incorporate individual photographs into a photomosaic correctly.  The best results had been obtained by using photogrammetric techniques to create orthographic elevation line drawings for control backgrounds for image placement.  Geomatics, or surveying, would be used to generate the spatial data.  An electronic Total Station was used to capture accurate three-dimensional points on the facades of the buildings that were then processed to produce the necessary orthographic control backgrounds.  The surveying portion of the project started in 2005 and was completed during the 2006 season.

Equipment – The following items were used to accomplish the work[1]:

  • Leica TCR 307 Total Station, batteries, charger
  • Tripod
  • Standard and mini reflective prisms
  • Hand tape
  • Stadia rod
  • Motorola walkie-talkie set
  • Marking chalk
  • Laptop computer with Leica Survey Office and AutoCAD software, printer

    Field notes – Equipment lists, sketches, primary and temporary survey station coordinates, job numbers, collected point data and miscellaneous notes were recorded by date in a field journal.

    Work schedule – Several different daily routines were tested.  The most successful was to accomplish survey work in the morning and early afternoon, have lunch on site during the heat of the day, layout and plan the next day’s work in the late afternoon, and download survey data from the Total Station to a laptop computer in the evening.

    Crowds and safety – The large number of individuals walking along the street obscured surveying lines of site and accidentally or sometimes even maliciously bumped equipment.  Work was, therefore, started as early in the day as possible before the crush of visitors from bus tours and cruise ships arrived.  Orange safety cones and tapes were used to delineate a buffer space.

    Jobs and job numbers – Each city block was surveyed as a separate job and given a number corresponding to its regio and insula designation.

    170 survey forum dPrimary survey stations – Surveying equipment and techniques determine the location (coordinates) of unknown points by accurately measuring distances and angles from a known point.  In order to record the coordinates on the facades of the buildings on via dell’Abbondanza, it was necessary to establish a series of known points, called survey stations, along the street. A visual inspection of the street surface revealed a number of embedded metal point markers that remained from previous surveying and mapping projects.  However, the coordinates of the points were unknown.  Fourteen of the markers were selected along the length of the street to be used as primary survey stations.  A “witness sketch” was made of each so they could be located when required.

    231_coordinatesSurvey station coordinates – A traverse is a series of measurements that define the relative coordinates of consecutive points.  The survey point at the forum was assigned the coordinates of 0.00, 0.00, 20.00, which defined an east-west/north-south origin of zero and an elevation of 20 meters.  A closed traverse was then made of the fourteen survey points from the forum to the Sarno Gate and back to the forum.  A closed traverse starts and ends at the same location in order to provide a numerical quality check.  The resulting fourteen coordinates were then used as the primary survey stations for all subsequent project measurements.  The coordinates were recorded in the memory of the Total Station for future use.

    Temporary survey stations – The Leica TCR 307 is a reflectorless Total Station.  The instrument is able to calculate the coordinates of a point on a building by emitting a pulse of laser light.  This type of Total Station is capable of measuring the angle and travel time of the beam of light as it is reflected from the surface of the structure rather than from a mirrored reflector.  The buildings are not harmed in any way.  Unfortunately, the effective distance of this early-generation machine in bright sunlight off the rough stone and plastered walls was limited to about 20 meters.  The fourteen primary survey stations proved to be insufficient for complete coverage.  Additional survey stations were required.

    Intermediate points were located along via dell’Abbondanza, usually in doorways on opposite sides of the street.  Small natural inclusions in threshold stones proved to be ideal temporary survey markers.  Witness sketches of the locations were made for future reference, and the coordinates of the temporary points measured from one of the fourteen primary survey stations.  This extended the network of known coordinates.  The doorway locations were also less susceptible to interference from visitors when surveying the structures on the opposite sides of the street.

    Surveying sequence – Work was generally accomplished in the following order:

    • A city block was selected for survey and a new job was established in the field notes journal.
    • A survey station with known coordinates was located opposite the subject insula.
    • Any necessary sketches and survey station data were recorded in the journal.
    • The Total Station was placed on its tripod and located directly over the survey station.
    • The instrument was leveled, its height above the survey station measured and the machine turned on.
    • The job number, instrument height and coordinates of the survey station were entered into the machine (or located in the memory).
    • A backsight was taken to another survey station in order to orient the instrument.
    • The telescope was used to locate a specific target point on the building facade.
    • The Execute button was pressed for the machine to measure and record the point.
    • This task was repeated until the measured target points adequately defined the facade.
    • Any explanatory information that would benefit the production of the photomosaics was recorded in the field notes for later placement on the background drawing.
    • If required, the instrument was moved to a different location to measure additional points.

    Targeted points – The objective was to define enough points on the surface of a structure so that it could be satisfactorily drawn.  The Total Station telescope was used to locate and measure points on the sides, ground line, wall tops and door and window openings of the facade.  In addition, other diagnostic features were targeted such as the insula street signs, thresholds, protruding beams, edges of roof tiles and holes.  Door and window lintels proved especially useful because they form almost the only straight lines in Pompeii.

    Measurement interval – Points on reasonably linear building features were collected every one-half to one meter.  Many building corners and edges are irregular and points were collected at closer intervals of approximately 10 to 20 cm.  Small objects, such as the ends of roof tiles, required even closer spacing.

    Sighting issues - The magnification power of the telescope was so great that it was often difficult to determine exactly what was being viewed on nearby structures.  In these situations a tapered rod was used to pinpoint locations on the ground and a fingertip to define corners and edges. The end of the rod or fingertip was more easily detected in the telescope in order to locate target points.

    Reduced effective range – The measurement distance of the reflectorless Total Station was sometimes reduced by bright sunlight or very rough surfaces.  In these instances a small white card or stadia rod was held behind or next to the targeted point for better reflection.  Chalk was also used to mark points on irregular edges.  Surveying in the direction of the late afternoon sun was avoided not only because of reduced measurement range, but also to prevent accidental eye damage.

    Obstructions – Vegetation, restoration scaffolding and other anomalies sometimes made it impossible to survey corners, edges or features on a facade completely.  In these cases, sketches were made in the field notes so that approximate representations could be added to the drawings.

    205 aes computer dCompleted surveys – Individual jobs were kept in the internal memory of the Total Station until complete.  At that time the field data were downloaded via a serial cable onto a laptop computer.  The points were saved in .txt files (text) so they could be easily read, and .dxf files (drawing exchange format) for import into CAD applications.  The files were backed up on data CD’s.

     

    2005 surveys – Project planning documents and the primary survey station traverse were completed first.  Ten insulae were then surveyed during the season.  The earliest surveys captured fewer target points on the building facades than later surveys.  The earlier results were acceptable, but experience showed that better quality elevation drawings were produced when more target points were collected.

    2006 surveys – The final twenty-two insulae were surveyed during the season.


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    [1] Many thanks to The Anglo American Project in Pompeii for the loan of their Leica Total Station.