Excited by a 980 nm laser, upconversion emissions centered at 384 (${\mathrm{Er}}^{3+}\text{:}{{}^4\mathrm{G}}_{11/2}\to {{}^4\mathrm{I}}_{15/2}$) and 408 nm (${\mathrm{Er}}^{3+}\text{:}{{}^2\mathrm{H}}_{9/2}\to {{}^4\mathrm{I}}_{15/2}$) in ${\mathrm{Er}}^{3+}/{\mathrm{Yb}}^{3+}$ codoped polycrystalline ${\mathrm{CaWO}}_4$ were successfully obtained. The ${}^4{\mathrm{G}}_{11/2}$ and ${}^2{\mathrm{H}}_{9/2}$ states of ${\mathrm{Er}}^{3+}$ were verified to be thermally coupled levels, which can emit the shortest wavelength emissions for optical thermometry known so far. Based on the change of their intensity ratio with temperature, an excellent optical thermometer can be designed, which can provide accurate temperature measurement from room temperature up to 873 K. More importantly, the sensitivity achieved here is much higher than reported temperature sensors based on ${\mathrm{Er}}^{3+}$ green luminescence.