Export Map bbox conversion

[NicoVink]

I am trying to learn how to use Export Map through the REST api. I managed to get an image of the city of Gent in Belgium by using the following url:

http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer/export?bbox=3....

Now, I want to look at this same area on the NatGeo_World_Map, but all I have been getting is empty images. I assume this has to do with the units of the NatGeo_World_Map being esriMeters instead of esriDecimalDegrees, but I have found no way to (correctly) convert the bounding box to esriMeters. Does anyone know how to do the conversion (or if I'm going at this the wrong way, how to get the same area of Gent on the NatGeo server)?

[NicoVink]

I figured out what I was doing wrong. I had already tried specifying the spatial reference of the bbox, which was the correct thing to do, but as it turned out, the NatGeo map does not have the data for Gent at the scale I was using. I tried it on New York and got the results I wanted.