<PAGE>   1
                                                          EXHIBIT 2.1(a)


                               ADDENDUM NO. 1
                                      TO
                           STOCK PURCHASE AGREEMENT

        This addendum (the "Addendum"), dated June 30, 1995, by and among
Landmark Graphics Corporation, a Delaware corporation ("Landmark"), MGI
Associates, Inc., a Texas corporation ("MGA"), and the other persons and
entities whose signatures appear upon this Addendum (the "Former
Shareholders"), relates to and is intended to amend that certain Stock Purchase
Agreement, dated September 29, 1994, by and among Landmark, MGA and the Former
Shareholders (the "Master Agreement"), and evidences that, in consideration of
the mutual obligations and covenants set forth herein, the parties hereto agree
as follows:

                                  ARTICLE I
                                   GENERAL

       SECTION 1.1  RELATION TO MASTER AGREEMENT.  Capitalized terms used but
not defined herein are used as defined in the Master Agreement.  Except as
modified herein, the Master Agreement will remain in full force and effect in
accordance with the terms thereof.  In the event of any conflict between the
terms and conditions of the Master Agreement and the terms and conditions of
this Addendum, the terms and conditions of this Addendum will govern and
control.

       SECTION 1.2  AMENDMENT OF THE MASTER AGREEMENT.  The definition of
"Actual Operating Income" set forth in Section 3.1 of the Master Agreement is
hereby amended by adding the following sub-paragraph:

               "(d)   If for any Earnout Period (hereinafter referred to in
this sub-paragraph (d) as the "Current Earnout Period"), other than the Earnout
Period ending June 30, 1995, the Actual Operating Income of MGA (calculated
without regard to the provisions of this sub-paragraph (d)) is less than its
Minimum Operating Income and during the immediately preceding Earnout Period
(hereinafter referred to in this sub-paragraph (d) as the "Prior Earnout
Period") the Actual Operating Income of MGA (calculated without regard to the
provisions of this sub-paragraph (d)) was greater than its Targeted Operating
Income for the Prior Earnout Period, the Actual Operating Income of MGA for the
Current Earnout Period (calculated without regard to the provisions of this
sub-paragraph (d)) will be increased by an amount equal to the result of
subtracting the Targeted Operating Income of MGA for the Prior Earnout Period
from the Actual Operating Income of MGA for the Prior Earnout Period
(calculated without regard to the provisions of this sub-paragraph (d))."



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                                  ARTICLE II
                                MISCELLANEOUS

       SECTION 2.1  COUNTERPARTS.  This Addendum may be executed in multiple 
counterparts, each of which will be deemed to be an original and all of which 
will be deemed to be a single agreement.  This Addendum will be considered 
fully executed when all    parties have executed an identical counterpart, 
notwithstanding that all signatures may not appear on the same counterpart.

        SECTION 2.2  SEVERABILITY.  If any of the provisions of this Addendum
 are determined to be invalid or unenforceable, such invalidity or
 unenforceability will not invalidate or render unenforceable the remainder of
 this Addendum, but rather the entire Addendum will be construed as if not
 containing the particular invalid or unenforceable provision or provisions,
 and the rights and obligations of the parties will be construed and enforced
 accordingly.  The parties acknowledge that if any provision of this Addendum
 is determined to be invalid or unenforceable, it is their desire and intention
 that such provision be reformed and construed in such manner that it will, to
 the maximum extent practicable, be deemed to be valid and enforceable.



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      IN WITNESS WHEREOF, Landmark, MGA and the Former Shareholders have
executed this Agreement as of the date first written above.



                                           LANDMARK GRAPHICS CORPORATION

                                           By: WILLIAM H. SEIPPEL
                                               __________________________

                                             Its: Vice President, Finance
                                                  _______________________
    



                                           MGI ASSOCIATES.  INC.

                                           By: FRANK D. McCORDIE
                                               ____________________________
                                               Name:  Frank D. McMordie
                                               Title: President

                                           MUNRO ENGINEERING INC.

                                           By: RODERICK G. MUNRO
                                               ____________________________
                                               Name:  Roderick G. Munro
                                               Title: President

                                                 /s/  FRANK D. McMORDIE
                                               ____________________________
                                                      Frank D. McMordie
                                              
                                           Eppler, Guerin & Turner, Inc. as
                                           Custodian for the benefit of
                                           Frank D. McMordie

                                           By: MAX R. McCASKILL
                                               ____________________________
                                               Name:  Max R. McCaskill
                                               Title: Vice President



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/s/ ELIZABETH McMORIDE-SCHMIDT         Eppler, Guerin & Turner, Inc. as
_______________________________        Custodian for the Benefit of
Elizabeth McMordie-Schmidt             Stephen H. Jaffe

/s/ JENNIFER L. McMORDIE
_______________________________
Jennifer L. McMordie                   By: MARK L. McMORDIE
                                           ________________________________
                                           Mark L. McMordie        
                                           Name:  Mark L. McMordie 
/s/ CHRISTOPHER L. McMORDIE                Title: Vice President   
_______________________________
Christopher L. McMordie
                                       /s/ THEODORE S. ROZSA
                                       _____________________________________
                                       Theodore S. Rozsa
/s/ HELEN LINDSLEY
_______________________________
Helen Lindsley
                                       Vendanges Investments, Inc.

/s/ GORDON R. BAYSTROM
_______________________________        By: RODERICK G. MUNRO
Gordon R. Baystrom                         _________________________________
                                           Name:  Roderick G. Munro
                                           Title: President
Eppler, Guerin & Turner, Inc.
As Custodian for the Benefit of
Gordon R. Baystrom                     Calyx Investments, Ltd.

By: MAX L. McMORDIE                    By: IAN MILLS
    ___________________________            _________________________________
    Name:  Max L. McMordie                 Name:  Ian Mills
    Title: Vice President                  Title: Vice President

/s/ GUY M. HUMPHRIES, JR.              /s/ JAMES R. LOVETT
_______________________________        _____________________________________
Guy M. Humphries, Jr.                  James R. Lovett

Eppler, Guerin & Turner, Inc.          /s/ KIRK D. HANES
As Custodian for the Benefit of        _____________________________________
Guy M. Humphries, Jr.                  Kirk D. Hanes

By: MAX R. McCASKILL                   /s/ DEBORAH CLOSE
    ___________________________        _____________________________________
    Name:  Max R. McCaskill            Deborah Close
    Title: Vice President

/s/ STEPHEN H. JAFFE
_______________________________
Stephen H. Jaffe


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/s/ GRANT GENEREUX                       /s/ IAN LEITCH
_________________________________        _________________________________ 
Grant Genereux                           Ian Leitch



/s/ DOUG SPACKMAN                        /s/ ROBERT PATTERSON
_________________________________        _________________________________ 
Doug Spackman                            Robert Patterson



/s/ DAVE SAVELLE                         /s/ KEVIN ROBINSON
_________________________________        _________________________________ 
Dave Savelle                             Kevin Robinson



/s/ GREG ROY                             /s/ BRIAN JANZEN
_________________________________        _________________________________ 
Greg Roy                                 Brian Janzen



/s/ JIM BURGLIN                          /s/ PRESTON NALDER
_________________________________        _________________________________ 
Jim Burglin                              Preston Nalder



/s/ DAN BEWS                             /s/ RICHARD WARD
_________________________________        _________________________________ 
Dan Bews                                 Richard Ward



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