I am creating a small land use map, as an exercise for visual image interpretation. I created a shapefile and started digitising polyogons with the polygon tool. The problem is that some of my polygons are automatically simplified/generalised. Some of the vertices disappear as soon as I finish sketch, which makes my map not very accurate.

Then, I also noticed that merging the polygons simplifies them. I just select polygons and click merge, then I notice gaps between them, some of the vertices having disappeared.

I searched the ArcGIS help, as well as online, to see if there are any editing options that I could change to disable this automatic simplification / generalisation, but I could not find any clear answers, only info about tools that can be used to deliberately achieve this effect. I just want ArcMap to leave my vertices where I put them.

This is my first editing attempt in ArcMap 10.2. I haven't come across this problem in ArcMap 10.1.

EDIT: I am including some further information.

From my Data frame properties:

Map units: Unknown Units

Display: Decimal degrees

Current coordinate system: No coordinate system.

As background image for my polygon shapefile I am using a scanned true color photo (23X23 cm), scale 1:10,000, in the form of a tiff file, with the following properties:

Columns and Rows: 5959, 5573

Cell Size (X.Y): 4.2333418e-005, 4.2333418e-005

Spatial Reference: undefined

Regarding the tolerance, the only types of tolerance that I have set in ArcGIS so far are the "snapping tolerance" and the "sticky move tolerance". I assume that in your comments you are talking about the X,Y tolerance, about which the ArcGIS Help says it has a default value of 0.001 meters (?)

  • 1
    What are the spatial reference properties of the shapefile: coordinate system, xy/z/m resolution and tolerance values. Is it the same as the data that you're digitizing from? Is ArcMap using the same one?
    – mkennedy
    Commented Nov 26, 2014 at 17:03
  • Shapefile or file geaodatabase feature class?
    – Aaron
    Commented Nov 26, 2014 at 17:31
  • Shapefile already mentioned - see description.
    – Terra
    Commented Nov 26, 2014 at 18:44
  • The shapfile that I created has no defined coordinate system (it is not important to define one because it is just a remote sensing exercise in which I am supposed to learn to distinguish between crops and pastures etc.; anyway I don't see how the coordinate system could influence the editing behaviour...) I do not know about what the tolerance values might be, but all settings should be the default settings, because I have just installed ArcGIS 10.2 and not done any changes to the settings. Where should I check that? Thanks.
    – Terra
    Commented Nov 26, 2014 at 19:01
  • 1
    A spatial reference has a coordinate system but also has resolution values that define the precision of how values are stored, and tolerance values the define the precision when processed. I think you're running into a problem with those values. Like using decimal degree values when the SR has projected tol and res values (aka too coarse for degrees).
    – mkennedy
    Commented Nov 26, 2014 at 19:57

1 Answer 1


It is close to 0 percent likely this photo is in DD.

1) Define the spatial reference of the background image in ArcCatalog.

2) Ensure it aligns with some other dataset (say from arcgis online) in ArcMAP.

3) Create a blank polygon in ArcCatalog.

4) Define the projection of 3 in ArcCatalog.

5) Start a blank mxd in ArcMap.

6) Add (1) the image with the projection defined.

7) Add (3) the polygon with the projection defined.

8) Start editing and you should be good to go.

numbers 1 and 2 are key.

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