GIS Frequently Asked Questions

Who can get copies of the data?

Please refer to the Data Distribution Policy & Fees page for complete details about accessing our data.

What coordinate system does Calaveras County use?

Calaveras County has standardized on California State Plane Coordinates, Zone III, NAD83. All units are measured in feet. A standard for vertical measurements has not been adopted at this time.

Why doesn't the data line up?

The data in the GIS system comes from many different sources. Each of the layers was processed by different individuals using a variety of methods and original sources. The result is that sometimes the layers do not line up well with each other. For example, the road network does not always match the right-of-ways in the parcel database.

Currently, the Public Land Survey System (Township & Range) and the 2002 Aerial Photos serve as our base map. Where possible, other layers have been adjusted to this base. In some cases, however, it is not possible or feasible to adjust individual layers.

How was the Parcel layer created?

The Parcel layer was compiled from a number of sources. Digital data was provided by Public and Private organizations in Calaveras County. These files were projected to a common coordinate system and combined together. The remaining areas were created from the Assessor's map books. Each page was scanned and then projected to real world coordinates. A process called rubber sheeting was then used to fit all the pieces together. This process fits the data together and minimizes changes.

Upon receiving the 2002 Aerial Imagery, and completing the GPSed Road network, the Parcel layer was adjusted to match these higher accuracy data sets. The PLSS layer was also adjusted for higher accuracy at this time. The PLSS and the Parcel layers are managed in combination so that parcel boundaries that are coincident with PLSS boundaries stay coincident.

How was the Roads layer created?

All state highways and county maintain roads were GPSed by a California certified engineering firm to make up our original Roads layer. The certified accuracy of these roads meets the National Mapping standard of +/- 10 feet, with most roads being well within this standard. Additional roads have been added to this layer by county staff using the same methods and equipment as the consulting firm, or by digitizing the roads from the 2002 Aerial Photographs. The Roads layer is updated as needed on an ongoing basis.

How do I download data?

Many of the GIS data layers used by Calaveras County are available to the public. They can be downloaded from this website via our "Open Data Portal"

What software do I use with the downloaded data?

The data provided for download on this website is in a GIS-compatible format. The exact format varies and is dependent upon the original format of the source data. Generally speaking, vector data is provided in ESRI shapefile format and imagery is either TIFF, MrSID or JPEG format. These formats are readable by the ESRI suite of products that include ArcExplorer, ArcView, ArcEditor, and Arc/Info. For info on these products, please go to Other programs such as MapInfo and AutoCAD may also be able to read these formats.

Calaveras County is not able to provide data in alternative formats.

What is the format for parcel numbers?

Parcel numbers are used to uniquely identify a piece of property. The assessor parcel number, or APN, is a 12 digit number composed of 4 parts: bbbPPPpppxxx


Example Description Length
bbb Book Number 3 Digits
PPP Page Number 3 Digits
ppp Parcel Number 3 Digits
000 Not used, set to 3 zeros 3 Digits


The Parcel layer stores an eight character parcel number consisting of the characters in positions 2 through 9 (i.e. bbPPPppp); the leading "0" in the book number and the trailing three "0's" are dropped. Parcel numbers are stored in the database as a character field. Dashes or other separators are not stored in the database, although some GIS applications will allow the user to input parcel numbers with dashes. These are stripped out by the application before processing.