Sunday, 17 June 2012

Methods of survey

It is common practise to select the method of documentation of historic buildings, tools and other resources necessary for the project based on

  •            Purpose of the Survey
  •          Contextual factors and complexity of the historic building
  •          Time and Cost factors
  •          Technical know-how of the working team

In this project, however, the deliverables are fixed i.e. Hand survey documents for four monuments within a fixed time and cost. The clients are still unsure about the purpose of the survey and have left it on the consultants to arrive at a suitable method and output.
As mentioned in a previous section, documentation of historic buildings was only carried out prior to civil works to aid in forming estimates of work. Although in the last few decades, more and more documentation works have been carried out to aid conservation works; rarely has it been used for the same and more and more documents have ended up in the archives.
It remains a huge challenge for the consultants to choose the right method and deliver a product that meets the Purpose of the survey; also overcoming the various constraints on site.
This section is an attempt to discuss the methods of documentation that are being conducted.

Hand Survey methods

The consultants are documenting the plan of the historic building using

  •           Triangulation; to arrive at an overall profile
  •            Chain and offset method for the details


The clients, lacking the technical knowledge of documentation over looked the provision of scaffolding and other such necessary supports and since height of the structure remains a constraint (6ft to 11ft), the consultants had to resort to photo rectification in order to document the exterior and interior elevations. Adding to this is the fact that the site is struck by monsoons for most of the project period due to which outdoor works becomes close to impossible. The number of working hands being limited, photo rectification seemed to be the most suitable option in this case. The facades are more or less flat and are less complex which would help reduce errors in the rectified photograph.  In cases where the room width is too narrow for photography, the consultants prepare a photo mosaic which is tested for accuracy by actual measurements.
The photographs taken are being rectified using Adobe Photoshop CS4 by correcting the lens distortion.

Friday, 1 June 2012

Noting measurements

The lack of necessary instruments and power supply on site,  forced us to note measurements on site, later transferring them onto the CAD program. We, therefore, had to follow a system of noting measurements that was legible and simple; aiding us in future references. In the initial stages of working, we started by noting dimensions alongside the dimension lines as is the common practice. But the criss-crossing dimension lines and dimensions created confusion, leading to errors in draughting. (Refer to the following photograph)
It was therefore necessary to follow a simpler method and hence, we resorted to noting measurements by first naming the relevant points using alpha-numeric labels and noting beside the sketch as shown in the following drawing.

Later, an even simpler and efficient way of noting was opted for.
The points to which dimensions were being taken were first noted, under which the relevant dimensions were noted. The same can be understood from the following photograph. This method has proved to be the most efficient for us, under the circumstances.

Thursday, 31 May 2012

Working with Errors

A bad works-man blames his tool - but the aim of this section is not to blame tool but to look for solutions. Errors are inevitable but can be minimised. Its the errors that sometime make work interesting and challenging. At this juncture, the project has not proceeded beyond documenting and measured drawing the built profile (plan). Hence this discussion is limited to what has been observed till this phase and includes :-
  • Errors in establishing the Reference Plan
  • Errors from Instruments used 
  • Errors from Marking the Profile
  • Errors from Measuring, Recording and Transferring dimensions 

Friday, 27 April 2012

Errors in establishing the Reference Plan

The positioning of reference plane is crucial for accurate location of a building. It needs to be exterior to the building, critically located so that determinant points along the section plane can be drafted from it. In this case, the closest possible permanent points from which a reference plane external to the building that could be generated was located 18.0 meters away. As a result, 4 points inside the building were chosen as the corners of the quadrilateral (reference plan) and measurements for interconnected spaces were taken with respect to the same. Over the length of 25.0 meters, it was observed that the cumulative error as a result has been 150 mm. 

After a discussion of the approach with Ar P. Latorre, it was realized that the process theoretically was correct, but in practice, the changes of errors, including human errors, area higher.  Also it was realised that drafting smaller areas from a larger reference plane negates errors than the other way round. The Diagram below shows the reference grid suggested by Ar P Latorre (Figure-1) and that executed at Site (Figure-2)
Ar. Latorres recommendations were based on the possibility of external reference plane, which will be at this juncture a time consuming exercise being subjected to permission form the client. 

The consultants have now identified critical points (that include perfect corners, openings, significant niches or larger moulding) from which dimensions can be transferred to connected spaces. These points will be triangulated first and will serve as a reference grid for smaller interior details. The consultants have also in the meanwhile requested the clients to permit construction of bearing points within 3 meters from the built surface, which will serve as an external reference plane. 

On redrafting with the aforementioned approach, it has been found that the difference between the location of old and new points has been almost 40mm. 

Lesson learnt
  • Incase an external reference grid is not feasible or cannot be availed - draft only  critical points to determine the overall profile of the section plane. The other details, like the niches, mouldings etc, in this case, should be drafted thereon. 
  • The larger the grid the more correct it will be (perhaps).
  • Avoid making moulding or other details as the critical points except when unavoidable. In this case, as there were higher number of chipped and or broken corners, one had to take edges of mouldings as reference corners. 

Thursday, 26 April 2012

Errors from Instrument used

Knowing ones instrument is the first step to identifying possible errors and minimizing it. The consultants used the following combinations of instruments - first chalk to mark datum, nylon string to hold between 2 points on the plane and plumb bob, replaced by a spirit level to check horizontality of the marked datum line. Each set had its own limitation and implication on the errors in marking the datum line and thereon in drafting the profile of the section plane. 

Set one - A Plumb bob, String and Chalk - The plumb bob is probably one of the oldest tools at hand to check horizontal levels. Strings were held tort between two ends between which the level were to be checked with a plumb bob. Unfortunately, it was found that, due the wind, the plumb swung incessantly, leading to errors in reading. The use of this set of tools were abandoned in this case. 
However, if the distance between the points (where levels had to be checked) was lesser than 1 meter and the force of the wind was lower, perhaps the use of plumb to measure horizontality would have been effective. In case, it was in-effective. When verified using a spirit-level, considerable difference in level measuring up to 25 mm maximum was noted.

Set two - Spirit-Level, String and Chalk - A spirit level, at that point of time, was the most convenient replacement to the plumb-bob. Prior to procurement, the accuracy of the tool needs to be checked i.e., if the reference planes (top and side bases) are horizontal or not. In this case, while procurement, the tool was not verified for correctness and after commencement of work, the inclination of its reference planes were realized. 
Moreover, the spirit level gives correct reading within a certain range i.e. when the bubble remains within the 2 lines (Refer to Figure - Range within which reading of spirit level is considered correct). Since the reading remains affected by the judgement of the user, the  correctness of the horizontal level varies from person to person. 

Lesson Learnt
  • It is a must to calibrate instrument and identity its errors before commencing work. Incase a large area should be covered, a smaller area must be measured as a sample to identify the extent and impact of errors from the instrument.                                                            As calibration is critical to accuracy, we plan to first locate the correction position of  the  bubble of spirit-level in use at this point to avert any subjectivity in reading. As a result of erroneous marking, there is a discrepancy of 80mm over an area of 25m long by 18m wide (Ground Floor).                                                                                                                                       
  • Using a plumb bob is more difficult than it apparently seems, especially while using it for the first time. Hence, we feel that one must practice and verify the horizontality of the level acquired while marking with a plumb bob. Also, one needs to buy a plumb bob where the bob is heavy enough to negate wind action - a problem we faced. It is certain that plumb-bob will swing in windy areas. 
  • A soft chalk is an inappropriate tool to mark as it leaves a thick mark as well as it fades.  It was initially chosen as the marks made would be temporary. However, the thickness of the chalk line as well as the mark it left on the building on erasing (incase the same had to be rectified) created confusion. Also, it was consciously decided that string or the measuring tape were to be held from the top portion of the marking (Refer to Figure - Point to Mark and Measure from). 

Wednesday, 25 April 2012

Errors from markings Profile

A - Cumulative Errors - This is possible the most critical of the errors and is cumulative in nature. It affects the horizontality and accuracy of the datum line and building profile. While marking the datum using a spirit level and the string, only 2 points at a time could be  marked. So the method used, was marking a second point from a previously marked point, eg - from 1, '2' is marked and from '2', '3' is marked and so on. Also, due to the nature of the built profile, rarely a series of point could be marked from a single point or the main the primary base point. As a result apart from the first set of points in a space, other dimensions were all transferred using a reference point. The series of marking increased the probability of a cumulative error. While visually surveying the profile,before measuring, errors in horizontality of the datum line were noticed. 

B- Nature of Built profile - The built profile in parts were inaccessible, broken or narrow, especially at the corners, making it difficult to even hold the end of the string. As a result,  it was difficult to mark the datum line along such corners or areas or points accurately, affecting the overall level. 

Friday, 20 April 2012

Step 01 : Identifying approach, process and information to be collected

Our Project brief requires us to prepare manually measured drawings, the basis on which conservation work of the structure and fabric can be directed.

A - Preparation to Work - Reconnaissance survey was conducted of the building, whose base measured 25 mt   by 18mt, having 4 floors (12 spaces in the ground floor, 8 in the first floor, 5 in the second floor, 2 in the third floor and terrace). Based on the drawing provided by the ASI, the spaces were numbered as S1 to S-N, in an anti clock-wise manner (Refer to Photographs- Naming of Spaces and Phasing for Measurements) and a reference diagram was made prior to commencement of  work.  

Naming of Spaces and Phasing for Measurements

B - Identifying Tools - Given that the work was commissioned to only 2 people, a process that provided accurate information yet is time efficient has to be selected. Also, in the absence of standards tools - such as a laser-cross-hair, inclinometers, disto-meter, total station, auto-leveler, digital spirit-level, the method to be resorted had to be manual, with basic tools - spirit level, string, chalk and measuring tape. Therefore, the schedule of work and time-frame had to be revised.  

C - Identifying bearing points and measurements - It was decided by the consultants that the built profile (floor plans) would be drawn by triangulating from an established reference plane. It must be mentioned that the reference planes are normally established outside the structure to be measured. However, in this case, fixed bearing points available are more than 8 meters from the built profile. Over this length, measurements taken are erroneous due to sagging of the tape. Moreover, the site being overtly visited, it is impossible (also requiring permission) to erect a temporary bearing point which will last 6 months (the length of project time). As a result, it was decided that an internal-grid would be established, from which measurements for the internal profile would be acquired and later, from the elevations, the external profile would be drawn. 

D - Phases of work - First, the profile of the interior spaces would be drawn, thereon, smaller details, that measure less than 1 meter i.e. mouldings, interior of niches, details and the profile of the floor, would follow. (Refer to Photographs - Phases of Work - Measured drawing of Floor Plan along established Datum Line)

Phases of Work - Measured drawing of Floor Plan along established Datum Line
Following the profile of the floor, would be the external elevations. For this a vertical reference grid at 2 meters interval would be established and measurements will be taken by a combination of methods - i.e. by triangulating and from rectified photographs (Refer to Photograph- Vertical reference grid along the North Facade).  

Vertical reference grid along the North Facade

E - Tests and Assessments - Based on the reconnaissance survey, a list of tests were also recommended as necessary- 
  • Tell-tales along cracks, separation joints which are possibly active
  • Excavation along at 4 points to assess condition of foundation (before rain)
  • Tests to identify composition and condition of plaster, mortar, bricks, flooring material (at different level and points)
  • Noting of changes in moisture levels, rising dampness etc