Your
truelat1 and
truelat2 are set to the exact same value ($31.473$). While this works, it creates a "Tangent" projection that is most accurate only at that exact line.
Suggestion: To make the map accuracy more even across the whole state of Himachal, you could set truelat1 = 30.5 and truelat2 = 32.5. This spreads the "perfect" zones across the southern and northern parts of your domain.
Setting two different values for truelat1 and truelat2 (called the
Secant case) is a trick to make your entire map more accurate, rather than just one single line.
Here is the simple breakdown of why this is better for Himachal Pradesh:
1. The "Touch" vs. "Cut" Method
When you use a Lambert projection, you are basically placing a "cone" over the Earth.
- Tangent (One True Lat): Your current setting (31.473 for both) means the cone just touches the Earth at that one line. The map is perfect at that line, but as you move North toward Lahaul or South toward Sirmaur, the distortion increases quickly.
- Secant (Two True Lats): When you set them to 30.5 and 32.5, you are making the cone cut through the Earth. It enters at 30.5 N and comes back out at 32.5 N.
2. The "Sweet Spot" is Bigger
By using two lines, you spread the accuracy across the whole state:
- At 30.5°N: Error is 0%.
- Between 30.5° and 32.5°: Error is extremely low (slightly compressed).
- At 32.5°N: Error is 0%.
Because Himachal Pradesh is about 300 km tall (from south to north), using two lines ensures that the
Map Scale Factor stays much closer to
1.000 for the entire domain. If you only use one line, the top and bottom of your map will have higher scale errors.
3. Impact on your 1km Nest
Since you are going down to
1km resolution, you are trying to be very precise.
- If your map scale factor is 1.002 instead of 1.000 because of distortion, your 1km grid cell is actually off by 2 meters every kilometer.
- Over a 100km domain, that adds up to a 200-meter shift.
- In the steep valleys of Kullu or Mandi, 200 meters is the difference between being at the bottom of a river bed or halfway up a mountain slope.
Summary:
Setting truelat1 = 30.5 and truelat2 = 32.5 "brackets" the state of Himachal, ensuring that your high-resolution nests (3km and 1km) are sitting in the most mathematically stable part of the projection.
Does this make sense ?
@Ming Chen @William.Hatheway
