on field vs at field

Some examples from the web:

• Being able to initially evaluate project routing from our desk, we cut down on field time and reduce our costs.
• In
• house game this morning on field B at the Bernardini between the players in Rudi Garcia's squad and Alberto De Rossi's youngsters.
• There are two ways to produce pellets on field:
• The solutions offered by calzavara for urban sites take into consideration the detailed 'on field' experience gained, capable of satisfying the different project requirements.
• A warm thanks goes to Audio Team, and especially to Giovanni Gazzola, that gave us the chance to verify "on field" Monrio Asty performances.
• (x) US EX
• Farm on field: crop under contract with seed and harvesting at buyer's charge.

Some examples from the web:

• She's been reinstated as commander of operations at field division headquarters.
• All the proposals from partners are assessed both at field level by ECHO experts and at headquarters.
• So I gave him one last shot at field work, and I requested his participation.
• ensure exchange of information at field level on findings and ongoing or planned actions.
• All this progress has been most welcomed, but should be followed by concrete actions at field level and accompanied by the decisive steps in favour of democratic ownership, accountability and better aid quality outlined in this report.
• The whole process of the selection of proposals to be funded and the provision of funding takes place in a context of continuous dialogue and coordination with those organisations, both at headquarter and at field levels.
• Further, at field level, the Commission coordinates and exchanges information with the national authorities through its Delegations and, on more technical matters, through Commission's humanitarian and technical support personnel present in the field.
• The visits of the projects and detailed control of operational and financial reports are part of the incorporated control mechanisms both at field and headquarter levels.