Method open

ThirdParty/libproj/vtklibproj/src/grids.cpp:191–255

// ---------------------------------------------------------------------------

GTXVerticalShiftGrid *GTXVerticalShiftGrid::open(PJ_CONTEXT *ctx,
                                                 std::unique_ptr<File> fp,
                                                 const std::string &name) {
    unsigned char header[40];

    /* -------------------------------------------------------------------- */
    /*      Read the header.                                                */
    /* -------------------------------------------------------------------- */
    if (fp->read(header, sizeof(header)) != sizeof(header)) {
        pj_log(ctx, PJ_LOG_ERROR, _("Cannot read grid header"));
        proj_context_errno_set(ctx,
                               PROJ_ERR_INVALID_OP_FILE_NOT_FOUND_OR_INVALID);
        return nullptr;
    }

    /* -------------------------------------------------------------------- */
    /*      Regularize fields of interest and extract.                      */
    /* -------------------------------------------------------------------- */
    if (IS_LSB) {
        swap_words(header + 0, 8, 4);
        swap_words(header + 32, 4, 2);
    }

    double xorigin, yorigin, xstep, ystep;
    int rows, columns;

    memcpy(&yorigin, header + 0, 8);
    memcpy(&xorigin, header + 8, 8);
    memcpy(&ystep, header + 16, 8);
    memcpy(&xstep, header + 24, 8);

    memcpy(&rows, header + 32, 4);
    memcpy(&columns, header + 36, 4);

    if (xorigin < -360 || xorigin > 360 || yorigin < -90 || yorigin > 90) {
        pj_log(ctx, PJ_LOG_ERROR,
               _("gtx file header has invalid extents, corrupt?"));
        proj_context_errno_set(ctx,
                               PROJ_ERR_INVALID_OP_FILE_NOT_FOUND_OR_INVALID);
        return nullptr;
    }

    /* some GTX files come in 0-360 and we shift them back into the
       expected -180 to 180 range if possible.  This does not solve
       problems with grids spanning the dateline. */
    if (xorigin >= 180.0)
        xorigin -= 360.0;

    if (xorigin >= 0.0 && xorigin + xstep * columns > 180.0) {
        pj_log(ctx, PJ_LOG_DEBUG,
               "This GTX spans the dateline!  This will cause problems.");
    }

    ExtentAndRes extent;
    extent.isGeographic = true;
    extent.west = xorigin * DEG_TO_RAD;
    extent.south = yorigin * DEG_TO_RAD;
    extent.resX = xstep * DEG_TO_RAD;