How should I write a statement of interest about how my research will fill gaps in the field, when I don't know enough about the field yet? I'm applying to do a master's at Canadian universities. My background is Earth &amp; Environmental Sciences.

When writing a statement of interest, I am familiar with the various research in the field I want to go into, however, a professor told me I should talk about "gaps in the data and where my research could fill those gaps". While I believe that's a great suggestion, I don't feel like I know enough about different methodologies and their faults to write a good statement of interest. Should I keep it on the vaguer side and relate it to the professor/research group's common interests?

On a different note, my marks aren't the best, they're about the admission requirements/slightly above. I have a lot of work experience in the field, and TA'd quite a bit. I also did an undergraduate thesis that I'm quite proud of (and hope to publish soon). So I know that already sets me apart from my peers and may make up for my lower average.  

Within that framework, should I discuss my marks and explain how I am a better researcher/writer than I am at taking exams, or do I leave out grades completely?
This statement of interest is both a persuasive and reflective piece. You need to sell yourself as best as possible. Below is one good way to do that. 


  General advice: Sell yourself, but be honest. Be specific and concrete - a few detailed examples are better than lists of stuff you did (which goes in a resume if one is requested). Highlight experiences that suggest you are ready for graduate-level research. Relate your experiences and interests to the research going on in the department/program to which you are applying.


Paragraph 1 - State your intention (admission to M.S. program). Here is a good place to very quickly summarized your qualifications and to highlight your general research interests.

Paragraph 2 - Why the admissions/selection committee should give you a spot. This paragraph is the place to mention your experience, your TA activities, and your thesis. If you have a lot to say, group it by categories (like coursework, research, service/teaching) and make more than one paragraph. Do not worry so much about how your current experience is shaping you to contribute greatly to the field. Instead, focus on that thesis experience and what you learned from it. Do you think you are more prepared for graduate-level research than other undergrads might be because of this experience?

Paragraph 3 - This paragraph is an opportunity to be frank about anything that others may see as a limitation and/or weakness. Then you should describe how these limitations are not so serious or are counteracted by some additional experience/strength that makes you stand out. You can also describe how you are addressing these limitations so that they will go away. You should mention your grades in this section. Do not hide from them, but do not fixate on them either.

Paragraph 4 (optional) - If you have some unique story about how you because interested in a field, subfield, or research project. If your reason boils down to "it sounds neato", then incorporate this content into another paragraph. You can bring up your thesis again and discuss how you chose the topic and how the work has shaped your interests.

Last paragraph - Wrap-up. Restate your intent and summarize the key factors why they should accept you. This is a good place to write some specific wordage for the program you are applying to: "My undergraduate thesis was on the effects of X on Y, and I would like to continue study in subfield Z. Projects being undertaken by professors J, K, and L at Institution M are especially interesting to me because..." If you do this, you are showing that you did your homework about the program. You can also put this kind of stuff up front.