ALTA Survey Accuracy Standards
Most descriptions of ALTA survey accuracy stop at a number: 2 cm plus 50 ppm. The number is easy to quote and easy to misread. It does not mean your parcel is located within 2 cm of its true position on earth, and a survey that meets it perfectly can still put the boundary in the wrong place — a point the 2026 ALTA/NSPS standards make in so many words. This page explains what the tolerance actually governs.
The Tolerance, Precisely
§3.E.v: "The maximum allowable Relative Positional Precision for an ALTA/NSPS Land Title Survey is 2 cm (0.07 feet) plus 50 parts per million (based on the direct distance between the two corners being tested)."
Two components. The fixed part, 0.07 feet, is about 21 millimetres. The proportional part, 50 ppm, scales with the distance being checked — 50 millionths of the length, or about 0.05 feet over 1,000 feet. On a small urban parcel the fixed term dominates; on a large tract the proportional term starts to matter. The tolerance is unchanged from the 2021 standards: the official ALTA/NSPS redline shows §3.E.v carrying no markup at all.
Crucially it is a test between two corners. The standard does not say a corner must be within 0.07 feet of anything absolute; it says the measured relationship between adjacent corners must be that tight.
Why "Relative" Is the Load-Bearing Word
§3.E.i defines Relative Positional Precision as "the length of the semi-major axis, expressed in meters or feet, of the error ellipse of the line connecting the monuments or witnesses marking adjacent boundary corners of the surveyed property at the 95 percent confidence level."
Unpacked: any measured position has uncertainty, and that uncertainty is not the same in every direction — it forms an ellipse rather than a circle. The semi-major axis is the ellipse's longest radius, so the standard is measured against the worst direction of uncertainty, not an average. The 95 percent confidence level means the true value is expected to fall inside that ellipse 19 times out of 20.
The standards say this is "most commonly estimated by the results of a correctly weighted least squares adjustment of the survey", with a covariance-matrix alternative. Both are statistical methods, which is the point: precision here is estimated, not asserted. The 2021 standards said it "can be" estimated that way; the 2026 wording is firmer.
This is why comparing a plat against a consumer mapping app is not a meaningful test. That comparison asks about absolute position on a global datum. The ALTA tolerance asks whether your corners are correctly placed with respect to each other — which is what determines where your building sits relative to your boundary, and therefore what a lender and title insurer actually care about.
Precise and Wrong: The Distinction the Standards Draw
§3.E.iii is the most useful sentence in the measurement section, and almost never quoted:
"A boundary corner or line may have a small Relative Positional Precision because the survey measurements were precise, yet still be in the wrong position (i.e., inaccurate) if it was established or retraced using faulty or improper application of boundary law principles."
Precision is how tightly the surveyor can measure and report a position. Accuracy is whether that position is the legally correct one. They are independent. A surveyor can measure to a hundredth of a foot and still hold the wrong monument, misread a deed call, or ignore evidence of occupation — and the resulting plat will be beautifully precise and materially wrong.
The standards are explicit that Relative Positional Precision "is not a substitute for the application of proper boundary law principles". §3.D requires boundaries to be established or retraced "in accordance with appropriate boundary law principles governed by the set of facts and evidence found" — that judgement, not the measurement tolerance, is where survey quality lives.
Four Sources of Uncertainty, One of Them Controllable
§3.E.ii enumerates where boundary uncertainty comes from. Only the fourth is a measurement problem:
| # | Source | Per §3.E.ii | Controllable? |
|---|---|---|---|
| 1 | Monument evidence | The availability, condition, history and integrity of reference or controlling monuments. | No — weighed as evidence |
| 2 | Record ambiguity | Ambiguities in the record descriptions or plats of the property or its adjoining properties. | No — weighed as evidence |
| 3 | Occupation lines | Occupation or possession lines as they may differ from the written title lines. | No — weighed as evidence |
| 4 | Relative Positional Precision | The measurement uncertainty of the line connecting adjacent boundary corners. | Yes — but never eliminated |
The standards note that of these four, "only Relative Positional Precision is controllable, although, due to the inherent errors in any measurement, it cannot be eliminated." The first three must be "weighed as part of the evidence in the determination of where, in the surveyor's opinion, the boundary lines and corners of the surveyed property should be located."
That is the honest shape of a boundary survey: a professional opinion built on evidence, with one statistically controlled component. Buying a survey on price alone optimises the one part that is already capped by the standard, and ignores the three parts that are not.
What the Surveyor Must Do to Hit It
§3.E.iv requires that for any measurement technology or procedure, the surveyor must (1) use appropriately trained personnel, (2) compensate for systematic errors including those associated with instrument calibration, and (3) use appropriate error propagation and measurement design theory — selecting the proper instruments, geometric layouts, and field and computational procedures — to control random errors so the tolerance is not exceeded.
Note the emphasis on design. Meeting the tolerance is a matter of how the measurements are arranged and checked, not simply of owning good equipment.
When the Tolerance Is Legitimately Exceeded
§3.E.v anticipates it: "It is recognized that in certain circumstances, the size or configuration of the surveyed property, or the relief, vegetation, or improvements on the surveyed property, will result in survey measurements for which the maximum allowable Relative Positional Precision may be exceeded in which case the reason shall be noted" under §6.B.x.
Dense woodland, extreme terrain, or a corner buried under a building can all defeat the geometry a tight measurement needs. That is why §6.B.x requires a note on the face of the plat explaining the site conditions responsible. An exceedance with an explanation is the standard working as intended. An exceedance with no note is not.
A related disclosure lives at §6.B.xi: the plat must note any part of the boundary where physical access within five feet was restricted. If a neighbour refused entry or a structure blocked a corner, the plat should say so — read those notes before assuming a boundary was fully observed.
Where State Rules Take Over
§3.B: many states and local jurisdictions set their own surveying standards, and "where conflicts between the standards set forth herein and any such jurisdictional requirements and standards of practice occur, the more stringent must apply."
So the ALTA/NSPS tolerance is a floor, not a ceiling. Your state may demand tighter measurements, mandatory monument setting, or a recorded survey — see the regulatory notes on the state ALTA survey cost pages. §3.C adds that there may be unwritten regional standards of care defined by the practice of the "prudent surveyor" locally, which is a further reason to engage a surveyor who works in the jurisdiction routinely.
All section references are to the 2026 Minimum Standard Detail Requirements for ALTA/NSPS Land Title Surveys, effective 23 February 2026, adopted by ALTA on 8 October 2025 and NSPS on 17 October 2025. Quoted passages are reproduced from that document.
Frequently Asked Questions
How accurate does an ALTA survey have to be?
Section 3.E.v of the 2026 ALTA/NSPS standards sets the maximum allowable Relative Positional Precision at 2 cm (0.07 feet) plus 50 parts per million, based on the direct distance between the two corners being tested. The 50 ppm component means the allowance grows with distance: about 0.07 feet plus a further 0.05 feet across a 1,000-foot line. The tolerance is unchanged from the 2021 standards. Note that it governs the relationship between corners, not position on the globe.
What does "relative" mean in relative positional precision?
It means the standard measures the uncertainty of the line connecting two adjacent boundary corners, not the absolute position of either one. The 2026 definition is the length of the semi-major axis of the error ellipse of that connecting line at the 95 percent confidence level. Practically, what matters for a boundary is that corners are correctly positioned with respect to each other and to the improvements; where the whole parcel sits relative to a global datum is a separate question and is not what this tolerance controls.
My phone GPS shows a different location than the survey. Is the survey wrong?
Almost certainly not. Consumer and uncorrected satellite positioning is not the measurement the standards govern, and it is not how a boundary survey is checked. The ALTA/NSPS tolerance applies to the measured relationship between boundary corners, which surveyors establish with calibrated instruments, corrected observations and error-controlled procedures. Section 3.E.iv requires appropriately trained personnel, compensation for systematic errors including instrument calibration, and error propagation design specifically so the tolerance is not exceeded. A handheld position disagreeing with the plat tells you about the handheld device.
Can a survey be precise and still be wrong?
Yes, and the standards say so explicitly. Section 3.E.iii states that a boundary corner may have a small Relative Positional Precision because the measurements were precise, "yet still be in the wrong position (i.e., inaccurate) if it was established or retraced using faulty or improper application of boundary law principles." Precision is how tightly the surveyor can measure and report a position. Accuracy is whether that is the correct position, which is a question of boundary law and evidence. The standards are explicit that precision is not a substitute for the correct application of boundary law.
What if the tolerance cannot be met on my site?
The standards anticipate it. Section 3.E.v recognises that the size or configuration of the property, or its relief, vegetation or improvements, may produce measurements exceeding the maximum allowable Relative Positional Precision. Where that happens the reason must be noted on the face of the plat under Section 6.B.x. So an exceedance is not automatically a defective survey — but it must be disclosed and explained, and an undisclosed one is a real problem.
Did the accuracy standard change in 2026?
The numeric tolerance did not change. The official ALTA/NSPS redline shows Section 3.E.v carrying no markup: it remains 2 cm (0.07 feet) plus 50 parts per million. The definition in Section 3.E.i was rewritten. The 2021 text defined Relative Positional Precision as the uncertainty in the position of one monument relative to the position of the monument marking an immediately adjacent corner; the 2026 text defines it as the error ellipse of the line connecting the monuments or witnesses marking adjacent boundary corners. The 2021 standard said precision "can be" estimated by a correctly weighted least squares adjustment; the 2026 standard says it "is most commonly" estimated that way. The effect is a clarification of what is being measured, not a change in how tight the work must be.
Related Pages
Requirements verified against the official 2026 ALTA/NSPS Minimum Standard Detail Requirements (NSPS, effective 23 February 2026) on 2026-08-11.