Checking Multiple Imagery Sources
While editing, you may default to using one of the background imagery layers more than the others. Each may have different benefits, as well as limitations, depending on the situation and location. This is why it is often a good idea to check multiple layers while researching a point location.
While we ask that you try to determine the final point location using one of the two National Map Imagery options or NAIP, the others can be helpful, and sometimes necessary, in determining a structure’s correct location.
Many factors can influence how useful the imagery is and may include the date it was captured, the time of year (tree cover), resolution, and scale.
Take for example the following law enforcement point. The Georgia State Patrol Troop E Post 33 – Milledgeville Station moved to a new location. This building is new construction, and so it cannot be seen on several of the imagery layers because the capture date is older.
The National Map Imagery lacks information to help identify the location of the new station.
Although we can get a clearer and closer view using the NAIP imagery, there is still no evidence of the new building construction yet.
On the ESRI imagery, we see evidence of construction, but no building yet. If we found other evidence (e.g., news article or mention on official website) confirming the exact location of the new station, this might be enough evidence to place the point.
As noted in our user materials and volunteer feedback, commercial mapping services may be used as supplementary sources to verify information gleaned from TNM imagery and authoritative resources, but we do not endorse the use of any specific commercial mapping services.
The Bing imagery is similar to the ESRI imagery within our TNMCorps editor but doesn’t provide additional evidence to verify the new building location.
In this example, the Google Earth imagery is more recent and does show the new building. Using this as a confirmation of the construction we noted on the ESRI imagery background layer, we can place the point in the editor.
Note: it is very important to only use commercial mapping services as a secondary source to confirm information gathered from authoritative sources. This is important not only due to the copyright restrictions that cover many commercial mapping services, but also to avoid the errors often found in aggregate data.
In this example, checking multiple imagery sources allowed us to identify the correct location of a new law enforcement point. As a reminder when using any of the imagery options in the editor, make sure to zoom all the way in before placing the point.
Do you have imagery tips and tricks? Send them to us at nationalmapcorps@usgs.gov.