如何点击地图并在填充的表格中包含纬度和经度字段？

I have a form that a user needs to fill out where the location needs to be identified. I have latitude and longitude input fields on the form. I am also looking for the decimal lat and long. What I would like seems really simple. I just want to have a link that the user can click on that would popup a map (google or yahoo) and he could use the map to find the location and just click on it and that would automatically populate the two input fields for lat and long from the map. Has anyone done this before? I have a form that a user needs to fill out whe