Auto Level Survey Calculation Formula

What is an Auto Level Survey?

An auto level survey involves using an optical leveling instrument to measure vertical distances between points. The auto level automatically compensates for the Earth's curvature and atmospheric refraction, providing accurate elevation measurements.

The primary purpose of an auto level survey is to establish a horizontal plane and measure vertical differences between points. This information is crucial for:

• Creating topographic maps
• Establishing benchmarks
• Grading construction sites
• Ensuring proper drainage systems
• Verifying building levels

Calculation Formula

The fundamental formula for calculating elevation differences in an auto level survey is:

Where:

• Backsight Reading: The reading taken on a known benchmark or previously established point
• Foresight Reading: The reading taken on the point whose elevation is to be determined
• Instrument Height: The height of the leveling instrument above the reference datum
• Rod Height: The height of the leveling rod above the point being measured

The ± sign indicates that the instrument height and rod height can either add to or subtract from the difference between the backsight and foresight readings, depending on their relative positions.

How to Perform an Auto Level Survey

Step 1: Set Up the Instrument

Place the auto level on a stable tripod and level it using the three leveling screws. Ensure the instrument is centered over the point you're measuring.

Step 2: Establish a Benchmark

Select a known benchmark with a stable elevation. Take a backsight reading on this point using the leveling rod.

Step 3: Measure Foresight Points

Move the leveling rod to the point whose elevation you want to determine and take a foresight reading.

Step 4: Calculate Elevation Differences

Use the formula provided above to calculate the elevation difference between the benchmark and the foresight point.

Step 5: Verify Measurements

Repeat the process in the opposite direction to verify your measurements and ensure accuracy.

Example Calculation

Let's consider an example where we want to determine the elevation of a point relative to a benchmark:

• Backsight Reading: 1.250 m
• Foresight Reading: 0.875 m
• Instrument Height: 1.500 m
• Rod Height: 0.300 m

Using the formula:

This means the point is 1.575 meters higher than the benchmark.

Common Mistakes to Avoid

When performing auto level surveys, several common mistakes can lead to inaccurate results:

1. Improper Instrument Setup: Ensure the level is properly leveled and centered before taking readings.
2. Incorrect Rod Height: Always measure and record the height of the leveling rod above the point being measured.
3. Temperature Effects: Be aware of temperature changes that can affect the length of the leveling rod.
4. Parallax Error: Ensure the leveling rod is directly in line with the crosshair when taking readings.
5. Inadequate Benchmarks: Use stable, well-established benchmarks for accurate elevation references.