How different amperage could affect a rechargeable device?
The iPad and other iOS devices are very good about only charging from appropriate voltages and currents. In fact, the iPad even is programmed to detect and show a not charging message in the status bar when it is connected to a power source with current too low. This is most commonly seen when the iPad is plugged into a computer that only supplies the normal USB current of 500 mA.

In your specific case, one amp at 5.1 V will charge the iPad slower than normal but still provide a useful charge even when the iPad is in operation. Should you be consuming a lot of data, a lot of graphics, and using GPS location you could possibly use more current than the one amp charger supplies, so just watch your battery level while it's charging and you should be fine.