The modern web mapping services (WMS) not only replace the traditional map for people obtaining the location, distance, and area, but also inspire versatile location-based services (LBS). But how accurate can these measurement achieve? A map is a projection of the Globe which inevitably contains distortions. In addition to the distortions of map projection, there are two types of error included in measurements. One is human error during the process of clicking the point on the monitor; the other is systematic error on web map platforms. To assess the accuracy of measurements, three popular WMS in Taiwan are chosen for the experiments, e.g. Google Earth, ESRI ArcGIS Earth, and Taiwan Electronic Maps. We click points and measure distances on the three WMS and compare each of distance measurement to the same distance calculated by the geodesic formula. The results of experiment shows that the human error is less than 1m, and the distance measurement of every arc degree is less than 2m on all three WMS.