Blog · September 30, 2026
What Declination Does on an Old Plat
Declination is the angle between magnetic north and true north at a place and a date. Old plats and deeds sometimes give magnetic bearings. Using today’s phone compass on an 1880 magnetic bearing ignores more than a century of change in the magnetic field.
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Magnetic bearings are dated
A needle points along the local magnetic field, not at the geographic pole, and the difference changes with place and with year. A surveyor in 1880 who ran a line magnetic north wrote a bearing that was true only in relation to the needle that day, plus whatever the notes said about declination. Some notes say the bearing is magnetic, or they state a declination east or west. Others are silent, and silence is not a license to assume true north. If you take that 1880 course and follow it with a modern phone, you are following a different magnetic north plus the phone’s own poor compass. You will not walk the deed line. You may not even walk a parallel to it.
How to read the note
Look for the words magnetic, variation, or declination, and for a date. A statement that courses are magnetic, with a variation of so many degrees east, lets a surveyor relate the work to true north as of that date. You still do not add today’s declination as a DIY correction and stake a fence. The correction belongs inside a retracement that also honors monuments. Monuments, once found, control over a recomputed magnetic course. The declination note explains the courses. It does not outrank a called-for stone. If the plat gives both a magnetic bearing and a monumented line, and they disagree, you note the conflict. You do not rotate the lot on a kitchen table and move the garage.
Modern plats usually leave the needle behind
Most contemporary subdivision plats use a basis of bearing tied to grid north, geodetic north, or a record line, and they say so. Declination still matters when you are reading the old deed the new plat retraced. The new plat may even state the old magnetic bearing and the new grid bearing side by side. Those parentheses are a gift. Copy both. County GIS is in a projected coordinate system and is not a magnetic document at all. Platbookmapper links to it for parcel finding only. It does not compute declination, and a declination website, even a good one, does not survey your lot. Use the historical note, then leave the ground work to a surveyor if a line must be marked.
What to do next
- Find whether the courses say magnetic, true, or nothing.
- Copy any declination and the year if the note gives them.
- Do not follow an old magnetic course with a phone compass.
- Prefer recovered monuments over a recomputed needle bearing.
The mix-up to avoid
Adding today’s declination to an old magnetic bearing and staking the result. The field has changed, the phone is crude, and the monuments, not the needle, are what the retracement has to find.
Questions people ask
Is declination the same as the basis of bearing?
Declination is one physical fact about the magnetic field. The basis of bearing is the plat’s statement of its reference. A basis can be magnetic, and then declination matters. A basis can be grid north, and then declination is background.
Where do I get historical declination?
Official geomagnetic models publish estimates for a place and a year. They are estimates. They can help you understand an old note. They are not a survey, and they do not replace monuments.
Find the county map
The county GIS map is the practical place to start. Find it from the Platbookmapper directory, then treat the recorded plat and deed as the documents behind the picture.
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.