Parker Moreland did an excellent analysis on measuring g with a photogate and suggests a procedure for minimizing error. This article also includes a formula and chart for calculating a local value for gravitational acceleration.
“Improving precision and accuracy in the g lab.” Parker Moreland. The Physics Teacher, Sep 2000. http://dx.doi.org/10.1119/1.1321823
You can also get a theoretical local value for g from WolframAlpha, http://wolframalpha.com
Replace Beaverton, OR in this example with your own location: http://www.wolframalpha.com/input/?i=gravitational+acceleration+Beaverton,+OR