TRIMBLE NAVIGATION LIMITED
                                  EXHIBIT 10.57

                                FOURTH AMENDMENT
                                       TO
                           REVOLVING CREDIT AGREEMENT



        This FOURTH AMENDMENT (the  "Amendment"),  dated as of July 30, 1997, is
among Trimble Navigation Limited (the "Borrower"),  BankBoston,  N.A., f/k/a The
First National Bank of Boston ("BKB"), Mellon Bank, N.A. ("Mellon", and together
with BKB, the "Banks"),  and BankBoston,  N.A., f/k/a The First National Bank of
Boston as agent for itself and the other Banks (the "Agent").

        WHEREAS,  the  Borrower,  the Banks and the  Agent are  parties  to that
certain  Revolving Credit  Agreement,  dated as of August 4, 1995 (as amended by
the First Amendment to Revolving Credit  Agreement,  dated as of April 30, 1996,
the Second Amendment to Revolving Credit  Agreement,  dated as of June 30, 1996,
and the Third amendment to Revolving Credit Agreement, dated as of September 30,
1996, the "Credit  Agreement"),  pursuant to which the Banks, upon certain terms
and  conditions,  have made  loans to an may  issue  letters  of credit  for the
benefit of the Borrower; and

        WHEREAS,  the Borrower had requested that the Banks agree, and the Banks
have agreed,  on the terms and subject to the  conditions  set forth herein,  to
make certain changes to the Credit Agreement;

        NOW, THEREFORE, the parties hereto hereby agree as follows:

        Section  1.  Defined  Terms.  Capitalized  terms  which are used  herein
without  definition and which are defined in the Credit Agreement Shall have the
same meanings herein as in the Credit Agreement.

        Section 2.    Amendment of Credit Agreement.  The Credit Agreement is 
hereby amended as follows:

        (a)  Section  1.1 of the Credit  Agreement  is amended by  deleting  the
definition  of Maturity  Date  contained in such section 1.1 and restating it in
its entirety as follows:

               Maturity Date.       September 1, 1997.

        (b) Section 8.3 (q) of the Credit  Agreement  is amended by deleting the
amount  "$2,000,000"  contained  in such  Section 8.3 (q) and  substituting  the
amount "$4,000,000 therefor.

        Section 3. Affirmation and Acknowledgment of the Borrower.  The Borrower
hereby  ratifies and confirms all of its  Obligations  to the Banks,  including,
without  limitation the Revolving  Credit Loans, and the Borrower hereby affirms
its absolute and unconditional  promise to pay to the Banks the Revolving Credit
Loans and all other amounts due under the Credit Agreement as amended hereby.

        Section 4.    Representations  and  Warranties.  The Borrower  hereby  
represents  and warrants to the Banks as follows:

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        (a) The execution and delivery by the Borrower of this Amendment and the
performance  by the  Borrower  of its  obligations  and  agreements  under  this
Amendment and the Credit  Agreement as amended hereby,  are within the corporate
authority of the  Borrower,  have been  authorized  by all  necessary  corporate
proceedings  on behalf of the Borrower,  and do not an will not  contravene  any
provision  of law or any  other  the  Borrower's  charter,  other  incorporation
papers,  by-laws,  or any stock  provision  or any  amendment  thereof or of any
indenture, agreement, instrument or undertaking binding upon the Borrower.

        (b) This Amendment and the Credit Agreement as amended hereby constitute
legal, valid and binding obligations of the Borrower,  enforceable in accordance
with  their  respective  terms,  except as limited  by  bankruptcy,  insolvency,
reorganization,  moratorium or similar laws  relating to or affecting  generally
the enforcement of creditors' rights.

        (c) No approval or consent of, or filing with, any  governmental  agency
or  authority  is  required to make valid and  legally  binding  the  execution,
delivery  or  performance  by the  Borrower  of  this  Amendment  or the  Credit
Agreement  as  amended  hereby,  or  the  consummation  by the  Borrower  of the
transactions  among the  parties  contemplated  hereby and  thereby or  referred
herein.

        (d) The  representations  and  warranties  contained in Section 6 of the
Credit  Agreement were correct at and as of the date made.  Except to the extent
that the facts upon which such  representations  and warranties  were based have
changed in the ordinary course of business  (which changes,  either singly or in
the aggregate,  have not been materially adverse) and after giving effect to the
provisions hereof,  such  representations and warranties also are correct at and
as of the date hereof.

        (e) The Borrower  has  performed  and complied in all material  respects
with all terms and conditions  herein  required to be performed or complied with
by it prior to or at the time hereof,  and as of the date  hereof,  after giving
effect to the provisions hereof, there exists no Event of Default or Default.

        Section 5.    Effectiveness.  The  effectiveness  of the Amendment  
shall be subject to receipt by the Agent of this Amendment executed by each of 
the Borrower, the Banks and the Agent.

        Section 6. Miscellaneous  Provisions.  (a) Except as otherwise expressly
provided by this Amendment,  all of the terms,  conditions and provisions of the
Credit Agreement shall remain the same. It is declared and agreed by each of the
parties hereto that the Credit Agreement,  as amended hereby,  shall continue in
full force and effect,  and that the Amendment and the Credit Agreement shall be
read and construed as one instrument.

        (b) THIS AMENDMENT IS INTENDED TO TAKE EFFECT AS AN AGREEMENT UNDER SEAL
AND SHALL BE CONSTRUED ACCORDING TO AND GOVERNED BY THE LAWS OF THE COMMONWEALTH
OF MASSACHUSETTS.

        (c) This  Amendment may be executed in any number of  counterparts,  but
all such counterparts  shall together  constitute but one instrument.  In making
proof of this Amendment it shall not be necessary to produce or account for more
than  one  counterpart  signed  by  each  party  hereto  by  and  against  which
enforcement hereof is sought.

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        (d) Pursuant to Section 15 of the Credit Agreement,  the Borrower hereby
agrees to pay to the Agent, on demand by the Agent, all reasonable out-of-pocket
costs and expenses  incurred or sustained  by the Agent in  connection  with the
preparation of this Amendment (including reasonable legal fees).

        IN WITNESS WHEREOF, the parties hereto have executed the Amendment as of
the date first written above.


                                            TRIMBLE NAVIGATION LIMITED

                                              By:/S/ John E. Huey
                                                Name: John E. Huey
                                                 Title: Treasurer


                                                  BANKBOSTON, N.A.
                                        f/k/a THE FIRST NATIONAL BANK OF
                                        BOSTON, individually and as Agent

                                              By: /S/ Teresa Heller
                                                 Name: Teresa Heller
                                                 Title: Director


                                                  MELLON BANK, N.A.

                                              By: /S/ Sean C. Gannon
                                                 Name: Sean C. Gannon
                                                 Title: Vice President



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