The letter is not the work, but it is the evidence of the work. If the actual engagement is professional surveying, the letter should say so plainly; if it is site supervision, the listing does not reach it.
Test the activity, then fix the wording
Start by asking what the person is being paid to produce. If the deliverable is survey data, plans, boundary determinations or the professional judgement attached to them, the engagement is professional work within the listed profession and the letter is simply badly drafted. Ask for a corrected version that describes the professional output first and treats coordination as incidental. If instead the deliverable is a completed site and the surveying is occasional, the honest conclusion is that the listing does not cover the assignment, and no rewording repairs that. A reviewer at a port of entry has the letter, the credential and a short conversation. Making those three agree is the entire preparation task, and it has to be done before travel rather than explained afterwards.
Hypothetical example: a hydrographic survey specialist is offered a role mapping a harbour channel, but the proposed letter calls the job field coordination. The first review compares the daily deliverables with the listed profession and the applicant's actual qualification. If maps and professional measurements are the product, the employer can describe that work accurately; if dispatching crews is the product, the parties must choose another plan. Keep the citizenship record and any ownership facts beside the letter. TN cannot be used for services to an American enterprise the applicant solely owns or controls, and a changed label does not settle that issue.