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

Step 02 : Establishing a Datum Line

The first step towards architectural measured drawings was to establish a datum line i.e. a section plane that becomes a horizontal reference plane at which the plan is drawn. Usually this horizontal plane is established at a level of 3 feet or 900mm from the floor level, explaining all recesses, projections and other details, that comprehensively reflect the building and its defects (like settlement, movement of members, inclinations etc). However, in the case of Karen Ghar, datum line was not established with respect to the the floor level, as the latter differed at different parts of the building. Also, a section at 3 feet would only provide partial information of the built profile, hence, the level was selected at an average of 920mm-1000mm from the floor that provided the profile with all openings, niches, recesses, mouldings etc.

Initially the marking was done using a string, white chalk and plumb bob, of which the latter was replaced by a spirit level (manual). The string was held tort at two ends – from a marked point on the building to a corner or point in the building to be marked. The level was checked using a spirit-level for horizontality and marked using the chalk. (Refer to Photograph - Marking the Datum Line along built profile)

Marking the datum line 

Step 03: Field Sketches

  For field sketches, the following are being used:

a.      Graph paper of A2 size (594 x 420 mm) with grids in centimetre
b.      A3 transparencies
c.      Drawing board of size A2
d.      Pencils with 0.5 mm leads
e.      Scale (30 cm)
f.       Cello-tape to secure graph paper         

  1. An indicative plan with spaces in the ground floor labelled S1, S2, S3….S15 is used for reference.
  2. The team has started with noting interior measurements. We are following an anti-clockwise order for measuring interior spaces.
  3. A proportionate sketch of the room plan, along the marked datum line is first drawn on the graph paper. On each of these sheets, details such as Structure code, Content, Date, erroneous points, corrections are being recorded.
  4. In our initial stages of noting measurements, the sketch plan included even the details (niches, moldings, archways etc) and dimension lines were drawn between every point in the room. But this method resulted in confusion, making it difficult to comprehend the sketch and causing errors while draughting. 
  5. As the team is severely short of working hands and time, it was not possible to draught manually on site. The noted points had to draughted off-site using Auto-CAD. We had to hence, adopt an easier yet effective way of noting measurements. It was later decided that we, first, label each point on the sketch plan alpha-numerically. The measurements between points are then noted beside the sketch itself. Although it is understood that this is not the technically correct method of recording, we have realised that this is the most effective method, under the given circumstances. Please refer to the following photograph for a sample work sheet.
  6.  On completion of the profile of the ground floor (exterior and interior), an A3 print out of each of the rooms will be taken to work on the details.
  7. computerized log of all the field sketches is being maintained and will be submitted, for any future references.

Step 4 : Noting measurements

Instruments used
·         30  m measuring tape with graduations of 2mm each
·         15 m metal tape
·         30 cm scale
·         Set squares
·         Torch light

1. Before we began noting measurements, the working team had decided that measurements over 3000mm span will be avoided in order to minimise errors due to sagging of the measuring tape.
2.  Care was taken that the tape was always held vertical (i.e. perpendicular to the floor line) in order to minimise errors.
3. Wherever there was chipped, broken or loose masonry on the corners, care was taken to place the measuring tape on the marked point.
4. As the tape we use for measurements have graduations of 2mm, we are cautious while noting measurements.
5.  More than 2 measurements were taken for critical points to aid in cross-checking.
6. One of the limitations that we face on site even today, is the absolute lack of light in 3 interior rooms. Adding to the limitation is the large of the room (>5m x 5m). 
7. All dimensions are being noted in Millimetres.

Thursday, 19 April 2012

Step 05 - Transferring Dimensions

Transferring dimensions collected must be a simultaneous process and completed on  site. Though it is better to draft manually, in this case it was directly drafted in AutoCAD2010. The first step was to make a proportionate sketch of the space to be measured, on a graph paper (millimeters). The sketch reflects only those points which help develop the profile of the space i.e. openings, prominent mouldings and corners. These points are then marked alpha-numerically and in order. Dimensions to be taken for drafting specific points are then identified. For critical points, especially those over 3.0 m in length, a second set of dimension are also taken as cross-checks.

Different methods were exercised to find an appropriate one to note dimensions. Initially, diagonals were drawn connecting two points between which dimensions are to be taken. The latter was noted in the form of 2.88.6 - i.e. 2 meters, 88 centimeters and 6 millimeters.  This system of noting, recommended by the HABS and HAERS systems proved to be cumbersome and confusing and hence restricted to room S1 alone. Thereon, dimensions were recorded as - 2886 mm and called out as 'Two Eight Eight Six' and noted on the graph paper as - 1- A : 2886m.

Recording dimensions at Site

The same dimensions as noted were transferred to AutoCad on site or at Office on return. Dimensions that proved erroneous were marked and verified either immediately or on the following day. 

On Autocad, the intersection of the arcs of dimensions are retained as a distinct layer - 'MD-Draft_Arc' and dimension lines as 'MD-Draft_Measurement Lines'. It has been decided that any point that provide the built profile would not be drafted in Autocad but be done manually, noting the undulations of the surface later. 

Transferring dimensions to AutoCad