# Measuring acreage from satellite image?

I am trying to use a ESRI basemap satellite image in order to obtain forest acreage values from a tax map parcel (image included). I have a parcel that contains farmland and forested areas, and I want to find out how many acres of forested land that specific parcel has. I tried the measurement tool in ArcMap but I am unsure of how accurate that is.

Is there any other way to do this?

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• Your question can mean a few different things. Why do you think the measurement tool is inaccurate? Is it due to the manual nature of clicking on the screen and you want an automated solution, or because you're not sure how accurate the reported number is, due to your chosen projection? Or, instead of "most accurate" are you more interested in the "recommended way to solve this problem"? May 7, 2018 at 17:35
• I believe it could possibly be inaccurate for both of those reasons, however I do not know. I believe that my "clicking" to determine the area is accurate enough, so I suppose its based more off of the projection. Either way, I am generally just looking for an alternative way of determining the acreage of the forested area if at all possible. May 7, 2018 at 17:40
• What location is it that you are trying to ? May 7, 2018 at 18:02

I would recommend looking into the standards and process that Esri has provided in terms of it's measure tool

The Measure tool lets you measure lines and areas on the map. You can use this tool to draw a line or polygon on the map and get its length or area, or you can click directly on a feature and get measurement information.

You can adjust the method of measurement to take in account of Planar or Geodesic

When measuring in a data frame with a projected coordinate system, the default measurement type will be Planar. This means that 2D Cartesian mathematics are used to calculate lengths. Planar measurements reflect the projection of geographic data onto the 2D surface (in other words, they will not take into account the curvature of the earth). Geodesic, Loxodrome, and Great Elliptic measurement types may be chosen as an alternative if desired.

As this response to the post points out How accurate is ESRI world imagery basemap in ArcGIS?.

It depends where you are in the world. The World Imagery basemap is assembled from imagery from many different sources, shot at different times and with different post-processing. I don't think there is a way to find out exactly how far off it is without comparing it against some other data which has a known accuracy.

If you use the Identify tool on the World Imagery layer, it will tell you a bit more about the imagery you're seeing. Sometimes you can use that info to track down more metadata about the imagery, including accuracy information:

Start with a polygon feature class or shapefile. Add a Parcel ID text field and an "Area" numeric field. Start an edit session and digitize the areas of interest. Give each polygon the Parcel ID that it resides in. When you're done digitizing, open the attribute table, right click the Area field and choose Calculate Geometry. Go through the menus and choose your area units, then click OK. It will calc all of your areas at once. At this point you could run some reports from the attributes or export the table for further manipulation and summarization in Excel. Just remember that the areas do not auto-caluclate so if you go back and edit the polygon geometry later, the area in the attribute table is not updated.