Engineering Surveying Week 4

Question 1

What is the true origin?

Answer 1

• True Origin = 2 degrees West, 49 degrees North

Question 2

Why does a false origin exist and how far away is this from the true origin and central meridian?

Answer 2

• So we don’t get negative Eastings
• 400km West of Central meridian
• 100km North of the true origin

Question 3

We use a projection scale factor to measure scale distortion, what is the formula for this?

Answer 3

Question 4

How do you perform a mean sea level correction?

Answer 4

Question 5

What 3 distances do we need to consider in mean sea level correction?

Answer 5

1. National Grid distance (NG) on the projection surface
2. Ellipsoidal (E) distance on the ellipsoid surface
3. Ground Surface (GS) distance at mean height h_m above mean sea level

Question 6

What happens when F<1?

Answer 6

• The ellipsoidal distance is longer than the national grid distance

Question 7

What happens if F>1?

Answer 7

• The ellipsoidal distance is shorter than the national grid distance

Question 8

What 3 techniques are used in setting out?

Answer 8

1. Horizontal control techniques
2. Vertical control techniques
3. Coordinate positioning techniques