A simple way of improving the Global Navigation Satellite Systems (GNSS) slant ionospheric correction from Vertical Total Electron Content (VTEC) models is presented. In many GNSS applications, a mapping function is required to convert from VTEC, which may be provided in Global Ionospheric Maps (GIMs), to Slant TEC (STEC). Typical approaches assume a single ionospheric shell with constant height, which is unrealistic, especially for low-elevation signals. To reduce the associated conversion error, we propose the Barcelona Ionospheric Mapping Function and its first implementation at northern mid-latitudes (BIMF-nml). BIMF is based on a climatic prediction of the distribution of the topside vertical electron content fraction of VTEC (hereinafter A mu (2)). BIMF is convenient to be applied since no external data are required in practice. To evaluate its performance, we use as independent reference the STEC difference (so-called dSTEC) values directly measured from mid-latitude dual-frequency Global Positioning System (GPS) receivers that have not been used in the computation of the VTEC GIMs under assessment. It is shown that the use of BIMF improves the GIM STEC estimation compared to the single-layer assumptions. This is the case for the mapping functions used by the International GNSS Service (IGS) and Satellite-Based Augmentation Systems (SBAS). This improvement is valid not only for the UPC GIMs, up to 15% for the year 2014, but especially for the GIMs of other analysis centers, such as those produced by CODE and JPL, up to 32 and 29%, respectively.