12.1 The Mississippi Coordinate System (§§ 89-6-1 to -19): Zones, Parameters & Grid-to-Ground
Key Takeaways
The Mississippi Coordinate System of 1983 uses transverse Mercator East and West Zones with central meridians 88°50′ W and 90°20′ W, scale 1:20,000 too small, and origin latitude 29°30′ N (§ 89-6-7).
The MCS'83 East Zone origin has coordinates N 0 m and E 300,000 m; the West Zone origin has N 0 m and E 700,000 m (§ 89-6-7).
Under § 89-6-11, coordinates may be recorded only if derived from an accurate connection to a third-order Class I (1:10,000) or higher National Geodetic Reference System station.
Mississippi law defines one meter as 3.280833333 1/3 U.S. Survey feet and requires at least ten significant figures when converting coordinates (§ 89-6-17).
The Mississippi Coordinate System of 1927 may not be used after December 31, 1999; MCS'83 has been the sole system since (§ 89-6-15).
The statute
The standards' reference to "SB 2131" (Laws 1991, ch. 462, effective January 1, 1992) is Mississippi's plane coordinate law, now §§ 89-6-1 through 89-6-19:
| Section | Rule |
|---|---|
| 89-6-1 | Adopts NGS plane coordinate systems as the Mississippi Coordinate System of 1927 (MCS'27) and of 1983 (MCS'83); divides the state into an East Zone and a West Zone by county; a survey crossing zones may use either zone for all points |
| 89-6-3 | Coordinates are two distances, N (Y) and E (X), in U.S. Survey feet for MCS'27 and meters or U.S. Survey feet for MCS'83 (feet option added 2006); they must conform to NGS-published control values |
| 89-6-5 | Defines MCS'27 |
| 89-6-7 | Defines MCS'83 |
| 89-6-9 | The names "MCS'27" and "MCS'83" may be used only for coordinates based on these systems |
| 89-6-11 | Recording requirement: no coordinates may be recorded on a plat or public record unless derived from an accurate connection to an identified, permanently monumented third-order, Class I (1:10,000) or higher station of the National Geodetic Reference System, under Federal Geodetic Control Committee (FGCC) standards |
| 89-6-13 | Coordinates are a complete, legal, and satisfactory description of a point's location if the connection follows the chapter and the Board's standards of practice. Where the point was previously described another way, coordinates are additional evidence, and conflicts are resolved by common rules of evidence. A recorded real-property description using coordinates must be tied to the existing land system |
| 89-6-15 | MCS'27 shall not be used after December 31, 1999 |
| 89-6-17 | 1 meter = 3.280833333 1/3 U.S. Survey feet; use at least ten significant figures when converting coordinates |
| 89-6-19 | Metes-and-bounds and lot-and-block descriptions remain permitted |
Zone definitions
| MCS'27 East | MCS'27 West | MCS'83 East (2301) | MCS'83 West (2302) | |
|---|---|---|---|---|
| Datum / ellipsoid | NAD 27 / Clarke 1866 | NAD 27 / Clarke 1866 | NAD 83 / GRS 80 | NAD 83 / GRS 80 |
| Projection | Transverse Mercator | Transverse Mercator | Transverse Mercator | Transverse Mercator |
| Central meridian | 88°50′ W | 90°20′ W | 88°50′ W | 90°20′ W |
| Scale on central meridian | 1 part in 25,000 too small | 1 part in 17,000 too small | 1 part in 20,000 too small (k₀ = 0.99995) | 1 part in 20,000 too small (k₀ = 0.99995) |
| Origin latitude | 29°40′ N | 30°30′ N | 29°30′ N | 29°30′ N |
| Origin coordinates | X 500,000 ft, Y 0 | X 500,000 ft, Y 0 | E 300,000 m, N 0 | E 700,000 m, N 0 |
In U.S. Survey feet, the MCS'83 false eastings are 984,250.000 ft (East) and 2,296,583.333 ft (West). The East and West Zones therefore use different false eastings. An easting near 2.3 million feet is West Zone, and one near 1 million feet or less is East Zone.
County assignments (§ 89-6-1). The East Zone has 44 counties: Alcorn, Attala, Benton, Calhoun, Chickasaw, Choctaw, Clarke, Clay, Covington, Forrest, George, Greene, Hancock, Harrison, Itawamba, Jackson, Jasper, Jones, Kemper, Lafayette, Lamar, Lauderdale, Leake, Lee, Lowndes, Marshall, Monroe, Neshoba, Newton, Noxubee, Oktibbeha, Pearl River, Perry, Pontotoc, Prentiss, Scott, Smith, Stone, Tippah, Tishomingo, Union, Wayne, Webster, and Winston. The West Zone has the other 38, including Adams, DeSoto, Hinds, Madison, Rankin, Warren, Washington, and Wilkinson.
Why transverse Mercator, and where scale equals 1
Mississippi is long north–south, so a transverse Mercator projection, with its distortion growing east and west of the central meridian, keeps each zone's distortion small. On the central meridian the scale factor is k₀ = 0.99995, a grid distance 50 ppm shorter than the ellipsoid distance. Away from the meridian the scale factor grows approximately as:
k ≈ k₀ × [1 + x² ÷ (2R²)]
where x is the distance east or west of the central meridian (easting minus the false easting) and R is the earth's mean radius (about 20,906,000 ft). Setting k = 1 gives x ≈ 0.0100 R, about 64 km (roughly 40 miles) from the central meridian. Inside those lines the grid is short (k < 1); farther out it is long (k > 1). Near the zone edges, about 45 miles from the central meridian, k is only about +15 ppm.
Grid versus ground
Two factors convert a horizontal ground distance to a grid distance:
- Elevation (ellipsoid) factor: EF = R ÷ (R + h), where h is the ellipsoid height. Since GNSS gives h directly and leveling gives orthometric height H, use h = H + N. N is the geoid height, which is negative throughout Mississippi (roughly −25 to −30 m); take the value from the current NGS geoid model (GEOID18 for NAD 83(2011)).
- Grid scale factor: k, from the projection.
Combined factor: CF = k × EF. Then grid distance = ground distance × CF, and ground distance = grid distance ÷ CF.
Worked example (West Zone)
A project in Rankin County is referenced to MCS'83 West Zone, U.S. Survey feet.
- Mean easting E = 2,380,000.00 ft, so x = 2,380,000.00 − 2,296,583.33 = 83,416.67 ft east of the central meridian.
- Orthometric height H = 480.00 ft (NAVD 88); geoid height N = −95.20 ft; so h = 480.00 − 95.20 = 384.80 ft.
- R = 20,906,000 ft.
| Step | Computation | Result |
|---|---|---|
| Elevation factor | 20,906,000 ÷ (20,906,000 + 384.80) | 0.99998159 |
| Scale factor | 0.99995 × [1 + 83,416.67² ÷ (2 × 20,906,000²)] = 0.99995 × 1.00000796 | 0.99995796 |
| Combined factor | 0.99995796 × 0.99998159 | 0.99993955 |
| Ground 1,845.65 ft → grid | 1,845.65 × 0.99993955 | 1,845.54 ft |
| Design grid 2,500.00 ft → ground | 2,500.00 ÷ 0.99993955 | 2,500.15 ft |
Staking 2,500.00 ft on the ground for a 2,500.00-ft grid dimension would leave the point 0.15 ft short. That one systematic error nearly uses up the entire Class A distance tolerance for the line (0.03 ft + 50 ppm × 2,500 ft = 0.155 ft), leaving essentially nothing for ordinary measurement error.
Convergence angle
Grid north is parallel to the zone's central meridian. True (geodetic) north converges toward the pole, so the two differ by the convergence angle γ ≈ Δλ × sin φ, where Δλ is the longitude difference from the central meridian and φ is the latitude. East of the central meridian, grid north lies east of true north and geodetic azimuth ≈ grid azimuth + γ; west of it, the sign reverses. Example: 0°15′ east of the central meridian at 32°18′ N gives γ ≈ 15′ × sin 32.3° ≈ 8.0′. Rule 7.3 requires the convergence angle on any plat where State Plane Coordinates were used (Section 3.4).
Plat and description requirements
When State Plane Coordinates are used in a survey, Rule 7.3, item 18 requires the plat to show the zone, the datum, the method (including published control used), a combined or correction factor, the convergence angle, and coordinates of at least two reference points. A description that uses coordinates must include the same information (item 13). § 89-6-13 requires such a description to be tied to the existing land system: the PLSS or a recorded plat, as Rule 7.3's commencing-point and point-of-beginning rules also require. A good plat note states whether distances shown are ground or grid and gives the realization, for example NAD 83(2011) epoch 2010.00.
Looking ahead
NGS is modernizing the National Spatial Reference System, replacing NAD 83 and NAVD 88 with new terrestrial and geopotential reference frames and designing a new State Plane Coordinate System of 2022. Mississippi's statute still defines MCS'83 in meters and U.S. Survey feet. Until the Legislature amends chapter 89-6, legal descriptions and plats that use state plane coordinates follow the MCS'83 definitions above.
Which set of parameters defines the Mississippi Coordinate System of 1983 East Zone under § 89-6-7?
Lambert conformal conic; central meridian 89°00′ W; E 500,000 m
Transverse Mercator; central meridian 90°20′ W; scale 1:20,000 too small; E 700,000 m
Transverse Mercator; central meridian 88°50′ W; scale 1:25,000 too small; X 500,000 ft
Transverse Mercator; central meridian 88°50′ W; scale 1:20,000 too small; origin 29°30′ N; E 300,000 m
A surveyor wants to show State Plane Coordinates on a plat to be recorded in the chancery clerk's office. Under § 89-6-11, what connection must the coordinates have?
An accurate connection to an identified, permanently monumented third-order Class I (1:10,000) or higher NGRS station, under FGCC standards
Any handheld GPS reading averaged for at least one hour
A tie to the nearest section corner, regardless of its coordinates
A tie to the county's GIS parcel layer
A line measures 1,250.00 ft on the ground. The elevation factor is 0.9999820 and the grid scale factor is 0.9999580. What is the grid distance?
1,250.18 ft
1,249.93 ft
1,250.07 ft
1,249.65 ft
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