Blog · September 30, 2026
What NAD27 and NAD83 Mean on a Plat
NAD27 and NAD83 are different geodetic datums, different reference frames for latitude and for state plane coordinates. The same numbers in the wrong datum can miss by tens of feet. A plat has to name the datum. A parcel screenshot will not.
Platbookmapper is an Amazon Associate. If you buy through an Amazon link on this page, we earn a commission. The link is a product, not the county map.
Two reference frames
A datum is the agreed shape and position of the earth used to express coordinates. NAD27 is the older North American Datum of 1927. NAD83 is the newer North American Datum of 1983, and it has later realizations that matter to surveyors even when the plat only says NAD83. They are not the same grid shifted by a neat statewide foot. The difference varies by place. In some regions it is a few feet. In others it is much more. If a 1960 plat lists state plane coordinates on NAD27 and you drop those numbers into a NAD83 GIS, the point moves. The lot did not move. The frame changed. Bearings and distances tied to monuments can still describe the lot perfectly while the coordinates are in the old frame.
What the sheet should say
Look for the datum in the coordinate note, next to the zone, such as a named state plane zone, NAD83, in U.S. survey feet. If the plat gives only bearings and distances and no coordinates, the datum may be irrelevant to your reading. Do not invent NAD83 because it is newer. If a later survey says it transformed the old coordinates, read the method rather than subtracting a rule-of-thumb shift you found online. Rule-of-thumb shifts are how corners get doubled. Monuments control when the plat says they do. A transformed coordinate that misses a found iron by the datum shift is a processing clue, not a reason to slide the iron.
The web map is not a datum lesson
County GIS is displayed in whatever datum and projection the county chose. It may have transformed its parcels from an older file. The transformation can be excellent for assessment and still be feet away from a survey monument. You cannot use the web map to test NAD27 against NAD83. Platbookmapper links to the web map and does not transform datums, draw lot lines, or show zip codes. A phone’s location is typically a satellite position in a modern global frame, not NAD27, and it is not a survey. If you need a corner’s relationship to a published coordinate, a surveyor occupies the monument. You do not average a phone dot and a GIS vertex and call the average the datum shift.
What to do next
- Read the datum and zone off the plat before using any coordinate.
- Do not paste NAD27 numbers into a NAD83 project.
- If the plat has no coordinates, do not assign it a datum.
- Let a stated monument-control note outrank a transformed coordinate.
The mix-up to avoid
Subtracting a single famous shift, such as a hundred feet in some direction, in every county. Datum differences are local. A borrowed shift puts the point in a third place that is neither datum.
Questions people ask
Is WGS84 the same as NAD83?
They are close in everyday speech and not identical for careful survey work, and later realizations diverge. A phone is not expressing your lot in the plat’s datum. Do not type a phone latitude into a deed.
Will a datum note change my deed distances?
No. Distances and monuments in the deed stay as written. The datum note explains coordinates. If your deed has no coordinates, NAD27 and NAD83 do not rewrite a single course.
Find the county map
Open Platbookmapper and pick the county that taxes the land. Use that viewer to get the parcel id, then match it to the recorded plat if you need the easements and lot numbers.
Not a survey. County GIS and tax parcel maps are assessment and planning layers. They are not a substitute for a recorded plat, deed, or licensed survey.