Civil PE Exam - Water Resources - How to Use the Thiessen Polygon Method for Rainfall

Summary

In this Civil Engineering Academy video, Jason demonstrates the Theissen Polygon method to calculate average rainfall precipitation. He explains the formula involving the summation of rainfall data (pi) times the representative area (ai) of each gauge, divided by the total area. The video walks through connecting adjacent rain gauges, splitting areas between gauges, and drawing bisector lines to determine the representative areas. The method helps in calculating the weighted average rainfall, where larger areas contribute more to the final value.

Takeaways

• 🌧️ The video is about calculating average rainfall using the Theissen Polygon method.
• 📊 The formula for average rainfall precipitation (P_av) is the sum of (Pi * Ai) divided by the sum of Ai, where Pi is the rainfall data and Ai is the representative area of each rain gauge.
• 🔗 The representative area serves as a weighting factor to calculate the weighted average of rainfall data from different rain gauges.
• 📏 The first step in the Theissen Polygon method is connecting adjacent rain gauges with red dashed lines.
• 📐 The second step involves splitting the area between two gauges by drawing a perpendicular bisector line from the halfway point between the gauges.
• 🔵 The bisector lines are drawn in blue to differentiate them from the red dashed lines.
• ✅ Ensuring the bisector lines are at 90 degrees is crucial for accuracy.
• 🔼 The intersection point of the bisector lines helps in identifying the areas that belong to each rain gauge.
• 📈 The results from the Theissen Polygon method can be used to determine the representative area for each rain gauge, which is then used in the formula to calculate the average rainfall precipitation.
• 👨‍🏫 The presenter, Jason, aims to help viewers understand the Theissen Polygon method through this tutorial.

Q & A

What is the main topic of the video?

The main topic of the video is how to use the Thiessen polygon method for calculating the average rainfall precipitation.

What is the formula for calculating the average rainfall precipitation using the Thiessen polygon method?

The formula for calculating the average rainfall precipitation (Pav) is: Pav = (Σ Pi * Ai) / (Σ Ai), where Pi represents the rainfall data collected by each rain gauge and Ai represents the representative area associated with each rain gauge.

What is the purpose of the representative area in the Thiessen polygon method?

The representative area serves as the weighting factor in the weighted average calculation of rainfall. The larger the area, the more the data from that rain gauge contributes to the final average value.

What is the first step in applying the Thiessen polygon method?

The first step is to connect the adjacent rain gauges with dashed lines.

How do you split the area between two rain gauges in the Thiessen polygon method?

To split the area between two rain gauges, identify the halfway point between the two gauges, and then draw a line perpendicular to the dashed line connecting them, passing through the halfway point.

What is the purpose of drawing perpendicular bisectors in the Thiessen polygon method?

The perpendicular bisectors split the area between the adjacent rain gauges, helping to determine the representative area for each gauge.

What should you ensure when drawing the bisector lines?

When drawing the bisector lines, ensure that they are perpendicular (at 90 degrees) to the dashed lines connecting the rain gauges and that they intersect at a common point.

What do the different areas represent once the bisectors are drawn?

Each area represents the region that contributes to the rainfall data collected by its corresponding rain gauge. For example, the area on the upper left belongs to rain gauge A, and so forth.

How does the size of the representative area affect the calculation of average rainfall?

The size of the representative area determines the weight of the corresponding rain gauge data in the final average rainfall calculation; a larger area means a greater contribution from that gauge.

What can you do with the results obtained from the Thiessen polygon method?

The results obtained from the Thiessen polygon method can be used to calculate the representative area for each rain gauge, which can then be used in the formula to find the average rainfall precipitation.