Civil Surveying Flashcards

Differences between the projected contour gradient uphill side and the actual road gradient include

Answer: Excavation on the centre line

When the projected contour gradient on the uphill side of a proposed road differs from the actual road gradient, it indicates a discrepancy between the natural ground level and the design level. If the natural ground is higher than the desired road profile, material must be removed from the center line to achieve the correct gradient. This process of removing earth is known as excavation.

A plane table's "fix" from three known locations is good if

Answer: Middle station is nearest

In plane table surveying, determining a point's position (a 'fix') from three known stations involves drawing rays from each station towards the unknown point. A 'good fix' is achieved when the middle station among the three known points is nearest to the unknown point. This geometric arrangement minimizes the 'triangle of error' formed by the intersecting rays, leading to a more precise and reliable location for the fixed point.

Using a chain line, you may create a 45° angle with

Answer: French cross staff

A French cross staff is a specialized surveying instrument used to set out specific angles in the field, particularly multiples of 45 degrees. Its design, often octagonal with sighting slits, allows surveyors to accurately establish angles such as 45°, 90°, and 135° directly. This makes it suitable for creating a 45° angle with a chain line, unlike other cross staffs that typically only set out 90° angles.

Which of the following errors/mistakes may add up to + or -?

Answer: Erroneous length of chain

An erroneous length of chain is a systematic error in surveying, meaning it consistently affects measurements in a predictable direction. If the chain is too long, all measured distances will be recorded as shorter than their actual length, leading to a cumulative negative error. Conversely, if the chain is too short, measured distances will be recorded as longer, resulting in a cumulative positive error. This type of error can therefore add up as either positive or negative depending on the chain's deviation.

Correction for a chain of 100 links with a rise of 1 unit every n horizontal units is

Answer: 100/n2

The correction for slope in chaining, often called hypotenusal allowance, accounts for the difference between the measured inclined length and the true horizontal length. For a slope where there is a rise of 1 unit for every 'n' horizontal units (i.e., a slope of 1 in n), the approximate correction for a measured length L is given by L * (slope)^2. If the slope is 1/n, the correction is L * (1/n)^2 = L/n^2. For a chain of 100 links, the correction becomes 100/n^2.

Grid lines are perpendicular to

Answer: Line representing the central true meridian of the grid

In a grid coordinate system, grid lines are established to create a rectangular framework for surveying and mapping. The Y-axis of this grid is typically aligned with a designated central true meridian. Consequently, the X-axis (or the east-west grid lines) are constructed to be precisely perpendicular to this central true meridian, forming the orthogonal grid system.

Number of subdivisions per metre length of a levelling staff is

Answer: 200

A standard levelling staff is typically graduated to allow precise readings for elevation measurements. A meter length on the staff is commonly divided into 10 decimeters, each decimeter into 10 centimeters, and each centimeter into 10 millimeters. However, the smallest readable subdivision is often 5 millimeters, with alternating color patterns (e.g., red and white) marking these intervals. Therefore, a one-meter length (1000 mm) contains 200 such 5-millimeter subdivisions (1000 mm / 5 mm = 200).

Included angles in a precise traverse are determined by setting the vernier

Answer: Indefinite contour gradients are possible

This question appears to have mismatched options for the given prompt. Assuming the question intended to ask about contour gradients based on the correct answer, 'Indefinite contour gradients are possible' is correct because a contour line represents points of equal elevation. The gradient (slope) at any point on a contour is perpendicular to the contour itself, and its steepness can vary significantly along the contour's path, depending on the terrain and proximity to other contours.

A dumpy level was placed halfway between the 50-meter-distance pegs A and B, and the staff readings at "A" and "B" were 1.22 and 1.06, respectively. The readings at "A" and "B" with the level set to "A" were 1.55 and 1.37, respectively. The collimation error for a sight line of 100 meters is

Answer: 0.04 m inclined downwards

First, calculate the true difference in level (TDL) from the halfway setup: 1.22 m (A) - 1.06 m (B) = 0.16 m. Next, calculate the observed difference in level (ODL) from the setup at A: 1.55 m (A) - 1.37 m (B) = 0.18 m. The difference between ODL and TDL is 0.18 m - 0.16 m = 0.02 m. Since ODL > TDL, the line of sight was inclined downwards. This error of 0.02 m occurred over the 50 m sight from A to B. Therefore, for a 100 m sight line, the collimation error is (0.02 m / 50 m) * 100 m = 0.04 m, inclined downwards.

A prismatic compass's graded circle's zero is situated at

Answer: South end

In a prismatic compass, the graduated circle is attached to the magnetic needle and rotates with it. For direct reading of bearings, the zero mark (0°) on this circle is conventionally situated at the South end. This allows the observer to directly read the bearing from North as the needle aligns itself with the magnetic meridian, and the South end of the needle (with 0°) is sighted along the line of observation.

The horizontal distances measured by tacheometer are adjusted for

Answer: All the above

Tacheometric measurements involve calculating horizontal distances and elevations from observed staff readings and vertical angles. To ensure accuracy, these measurements require several adjustments. These include corrections for the Earth's curvature and atmospheric refraction, which affect the line of sight, and slope correction to convert inclined distances to true horizontal distances. While less common, temperature can also influence instrument components and atmospheric conditions, making 'All the above' the most comprehensive answer for precise work.