Altitude Corrections

Altitude Corrections

The dip correction must be made to Hs (sextant altitude) to compensate for your height of eye above sea level. This correction, which is always subtracted from Hs, can be found in the altitude correction tables on the inside front cover of the nautical almanac. For now, however, simply refer to the reproduction of the table provided here. View table the table gives the height of eye in either meters or feet. Be especially careful when using this table because the coast guard problems may use either meters or feet.

EXAMPLES: click to view.

THE DIP CORRECTION FOR HEIGHTS OF EYE GREATER THAN 8.0 FEET THROUGH 8.6 FEET IS 2.8'

THE DIP CORRECTIONS FOR HEIGHTS GREATER THAN 20.1 FEET THROUGH 21.0 FEET IS -4.4.

THE DIP CORRECTIONS FOR 20.1 FEET IS 4.3' .

IF YOUR HEIGHT OF EYE IS 9.7 METERS, WHAT IS THE DIP CORRECTION? 9.7 METERS WOULD FALL BETWEEN 9.5, METERS AND 9.9 METERS IN THE TABLES, SO THE DIP CORRECTION WOULD BE -5.5.

THE DIP CORRECTION FOR A HEIGHT OF EYE OF 48'4 FEET IS -6.7'.

TRY THESE FOR PRACTICE AND CHECK THE ANSWERS BELOW:

1. HEIGHT OF EYE: 22.8', DIP CORRECTION: _______________________

2. HEIGHT OF EYE: 3.4 METERS, DIP CORRECTION: ___________________

3. HEIGHT OF EYE: 42.5', DIP CORRECTION: _____________________

4. SEXTANT READING: 19˚-56.4', INDEX ERROR 1.3 ON THE ARC, HEIGHT OF EYE: 30.4', CORRECTED READING: ______________________

5. SEXTANT READING: 24˚-16.5', I.C.: -0.8, CORRECTED READING: ___________________

View answers

After you correct Hs for index error and dip, you have apparent altitude Ha .it is used to find the last (thankfully!) Correction you will need. This correction compensates for the refraction of the light coming through the earth's atmosphere, among other things. There is no standard abbreviation for this correction, so we will abbreviate it alt. In the case of a sun shot, it also corrects for the fact that you are measuring the angle to either its upper or lower edge called a limb, rather than to its exact center. To find the alt corrections we'll use the altitude correction table from the nautical almanac, a portion of which appears here: View table notice how the table is divided into two halves of the year. Make sure you enter the table for the month of your sighting. Also notice that the corrections differ greatly depending upon whether you are using the upper or lower limb of the sun. You enter the table with your apparent altitude ha. Suppose that when you corrected Hs for i.e. and dip, the resulting ha is 12˚ 27.31. You sighted the sun's upper limb. The date of the sight is March 10. What is the ALT correction? The answer is -20.3. That correction, when applied to Ha, will give you the observed altitude Ho:

(Ha)         12˚-27.3′

ALT              -20.3

(Ho)        12˚-07.0′

Suppose you observed the lower limb View table of the sun on December 18. Ha is 14˚ 42.0'. What is the ALT correction?      Answer: plus 12.6'

If you are shooting a star, the ALT correction is found in a different part of the altitude correction table.

Because a star is just a dot of light, there is no upper or lower limb. Nor do we need to worry about the month. Just enter the table with Ha and find the correction.

Suppose you sight the star Spica on April 19. Ha is 10˚ 32.8'. View table what is the "ALT" correction? Answer: 5.1.

Suppose you sight the star Spica on January 23. Ha is 12˚ 15.1'. View table what is Ho?

Ha         12˚ 15.1'

ALT             - 4.4

Ho          12˚ 10.7'

For practice, find the correction for the following: View table

  BODY

MONTH

Ha
CORRECTION

1. SUN UPPER LIMB APRIL 27˚ 54.8' ____________

2. STAR JULY 18˚ 27.3' ____________

3. SUN LOWER LIMB MARCH. 38˚ 29.2' ____________

4. STAR JUNE 33˚ 07.6' ____________

5. SUN LOWER LIMB DEC 12˚ 18.1' ____________

Practice Problems

Here, View table, you will see the complete "altitude correction tables" as it appears in the nautical almanac. Use it to work the following problems.

Problem:

You observe the lower limb of the sun on November 15.

The sextant reading was 19˚ 22.6.

Index error was 2.4 off the arc.

Your height of eye was 32 feet.

What was the observed altitude? Ho

Solution:

Hs    19˚ 22.6'

I.C.        +2.4'

=       19˚25.0'

DIP       - 5.5

Ha = 19˚ 19.5'

ALT   +13 .5

Ho = 19˚ 33.0

    Here are some more for practice. Work them out on a separate sheet of paper using the format you have learned. The answers are below.

Find ho for these sun shots View table

MO. HS I.E. I.C. HT. OF EYE LIMB OBSERVED

1 OCT 47˚ 56.0' 1.9' ON THE ARC 26 FEET UPPER

2 APR 25˚ 12.6' (+) 2.0' 8 METERS LOWER

3 MAR 36˚ 49.9' 2.1' OFF THE ARC 45 FEET LOWER

4 SEP 58˚ 38.8' (-) 1 .6' 22 FEET UPPER

5 DEC 64˚ 17.5' 1.3' ON THE ARC 12 METERS LOWER

6 MAY 43˚ 56.4' 3 .6' OFF THE ARC 60 FEET LOWER

Find ho for these star sights View table

7 JUL 32˚ 08.0' (+) 1. 8' 18 FEET

8 AUG 22˚ 56.4' 2.7' ON THE ARC 9 METERS

9 SEP 47˚19.1' (+) 2.1 37 FEET

10 OCT 61˚ 22 .3' 1.6' OFF THE ARC 5 METERS

Answers

1. 47˚ 32.3'

2. 25˚ 23.6'

3. 37˚ 00.5'

4. 58˚ 16.3'

5. 64˚ 25.8'

6. 44˚ 07.5'

7. 32˚ 04.2'

8. 22˚ 46.1'

9. 47˚ 14.4'

10. 61˚ 19.5'

Next Topic, Time

1 Introduction	9 Altitude Intercept	17 Latitude by Meridian Altitude
2 Corrections to Sextant	10 Using Position Plotting Sheets	18 Latitude and Azimuth by Polaris
3 Altitude Corrections	11 Plotting Lines of Position	19 Running Fixes
4 Time	12 Summary of 1 thru 11	20 Time of Sunrise Sunset
5 Finding GHA and Declination	13 Finding Deviation or Gyro Error	21 Star Identification/Selecting for Fix
6 Assumed Position and Local Hour Angle	14 Finding Azimuth by 229	22 Time Tick Problems
7 Computed Altitude and Azimuth	15 Amplitudes	23 Deviation Table Construction
8 Interpolation	16 Time of Local Apparent Noon	24 Sample Final Exam