A single description covering both phases, showing they are one professional assignment: the field program and its purpose, the interpretation and modelling that follow, the reports produced, the expected duration, and who within the employer directs and receives the work.
Write one letter, not two job descriptions
Splitting the letter into a field section and an office section can suggest two different roles, one of which may not be professional on its own. A better structure states the professional objective first, then explains that the applicant collects the data and then interprets it. Add the degree and any professional registration, and keep the phrasing consistent with the contract or scope of work the employer signed.
Hypothetical example: a soil scientist is retained to design a sampling grid across several parcels, oversee its execution, then produce the classification report a client's permit depends on. One paragraph naming that objective does more than a bulleted list of tasks, because it explains why the field days exist. Give the letter a stated end product and a named recipient inside the employer, so the assignment has a beginning and an end on paper.
Where the applicant holds a professional designation, state the awarding body and the standing of the registration as printed on the certificate rather than paraphrasing it. Compare the letter with the contract line by line: a scope of work describing deliverables the letter never mentions, or a duration the letter contradicts, is the kind of discrepancy that is easy to fix beforehand and awkward to explain afterwards.