What makes you and expert?

A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify as an opinion or otherwise, if:
(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles and methods; and
(d) the expert has reliably applied the principles and methods to the facts of the case.

What evidence requires expert testimony?
Any evidence that is scientific or technical in nature, such that the layperson, or juror would not have common knowledge or understanding.

The evaluation of expert, evidence and admissibility of each are based upon one of two:

This is the challenge (test) of the admissibility of the scientific evidence. Frye v. United States, 293 F. 1013 (D.C. Cir. 1923) states that an expert opinion is admissible if the scientific technique on which the opinion is based is “generally accepted” as reliable in the relevant scientific community.

This is the challenge (test) of the admissibility of the expert witness.
Daubert v. Merrell Dow Pharmaceuticals, which held in 1993 that Rule 702 of the Federal Rules of Evidence did not incorporate the Frye “general acceptance” test as a basis for assessing the admissibility of scientific expert testimony, but that the rule incorporated a flexible reliability standard instead;

Forensic Scene Mapping with Total Stations and 3D Laser Scanners General Electric Co. v. Joiner, which held that a District Court Judge may exclude expert testimony when there are gaps between the evidence relied on by an expert and his conclusion, and that an abuse-of-discretion standard of review is the proper standard for appellate courts to use in reviewing a trial court’s decision of whether it should admit expert testimony;

Kumho Tire Co. v. Carmichael, which held in 1999 that the judge’s gatekeeping function identified in Daubert applies to all expert testimony, including that which is nonscientific. The Judge is the Gatekeeper and has the task of weighing all evidence based on the challenges set forth by the prosecution or defense during a trial or sometimes, by the court itself.

The trial judge must ensure that the expert’s testimony is “relevant to the task at hand” and that it rests “on a reliable foundation.” Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 584-587.

A conclusion will qualify as scientific knowledge if the proponent can demonstrate that it is the product of sound “scientific methodology” derived from the scientific method.

Here are some of the factors considered:
1. Empirical testing: whether the theory or technique is falsifiable, refutable, and/or testable.
2. Whether it has been subjected to peer review and publication.
3. The known or potential error rate.
4. The existence and maintenance of standards and controls concerning its operation.
5. The degree to which the theory and technique is generally accepted by a relevant scientific community.
While each of the previous may or may not apply, they are general guidelines for the Judge to consider.

Compare states here at https://www.theexpertinstitute.com/daubert-v-frye-a-state-bystate-comparison/

Forensic Scene Mapping with Total Stations and 3D Laser Scanners Discussion for Admission of Laser Based Forensic Scene Mapping Evidence

Laser Scanning is based upon generalized scientific principles, all of which are accepted and recognized by the scientific community. Using elements of mathematics (geometry / trigonometry), physics and light, every measurement point can be quantified and proven accurate within a reasonable degree of scientific certainty.

1. The theory and technique are tested each time the equipment is used.
a. The base measurement theory/technique is tested and proven based on redundant observations; the distance and angular measurement precision are estimated in a least squares adjustment for comparison.
b. This is also accomplished through random positioning of a standard (scale bar) and comparing measurements and/or mathematical analysis of the geometric positioning of points to confirm measurement accuracy.

2. The theory and technique have been peer reviewed and accepted in both the Forensic Scene Laser/Mapping and Land Surveying Communities.
a. They are accepted as part of the official course content in such classes as Forensic Scene Mapping with Lasers, offered at the University of North Florida’s Institute for Police Technology and Management.

3. The maximum quantifiable error rate accepted in Forensic Scene Mapping:
a. Recommended for Small Crime Scenes
i. +/- º inch for most crime scenes, small areas
ii. http://www.nist.gov/forensics/upload/Crime-SceneInvestigation.pdf
b. Recommended for Crash Scenes
i. +/- 1 inch larger outdoor areas
ii. IPTM Forensic Scene Mapping Protocol

4. The standards are set forth by a mapping protocol, which outlines the setup and procedures for use. Since there is no official maintenance by the user, a factory calibration record is provided. The control is confirmed each time the device is used.
a. Accuracy Confirmation Measurements
b. Reference back to Item 1 above.
5. The theory and technique is widely accepted in the Crash/Crime Scene Reconstruction community.