> For the complete documentation index, see [llms.txt](https://docs.enercalc.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.enercalc.com/sel-faq/using-the-software/calculation-modules/loads-forces/seismic-loads/discrepancies-between-program.md). # Discrepancies between program results and USGS tool results Why ENERCALC seismic results occasionally differ from the USGS Seismic Design Maps tool. ## Why are there occasional discrepancies between program results and USGS tool results? The gridded USGS text files contain SS and S1 parameter values from the International Building Code. Please note that these values will not exactly match those provided by the U.S. Seismic Design Maps. The maps interpolate the underlying probabilistic and deterministic ground motions, as well as the risk coefficients, before calculating SS and S1. Should you choose to interpolate the gridded values provided by USGS, you will be doing so after the SS and S1 calculations have already been performed at the grid points. Proceeding with this order of operations will produce results that slightly differ from those calculated in the Design Maps. In addition, interpolation for a specific lat/lon coordinate requires a two-step interpolation. There must be an interpolation in the north-south direction, and then another interpolation in the east-west direction. If ENERCALC SEL interpolates in a different order than the USGS tool, the results could be different, even though both methods use a perfectly logical approach to the interpolation. Remember that if you feel more comfortable specifying an SS and S1 value from another source, that is always an option in ENERCALC SEL. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.enercalc.com/sel-faq/using-the-software/calculation-modules/loads-forces/seismic-loads/discrepancies-between-program.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.