Frequently Asked Questions (FAQs)

1) What coordinate system does Napa County use?
2) Who can get copies of the data?
3) Why doesn't the data line up?
4) How was the Parcel layer created?
5) How do I download data?
6) What software do I use with the downloaded data?
7) What is the format for parcel numbers?


1) What coordinate system does Napa County use?

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

2) Who can get copies of the data?

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

3) 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 1993 DOQQs 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.

4) How was the Parcel layer created?

The Parcel layer was compiled from a number of sources. Digital data was provided by each of the local cities. 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.

5) How do I download data?

Many of the GIS data layers used by Napa County are available to the public. They can be downloaded from this website or you can request a DVD with the data you need. Please note that there is a $24.00 per DVD charge to cover the cost of reproducing data on DVD. Data is also available on external hard drive for $225.00. Data downloaded electronically is available free of charge.

To download data, go to the Data Request Form . The first time you download, you will have to fill out the registration form completely. On subsequent visits, you only need to fill in the e-mail address that you registered with.

Once you have submitted your information, you will be taken to the Data Catalog. The data layers are grouped under several different headings. Click on the button next to each heading to display the list of layers. All layers that are available for download will have "Yes" listed in the Download column. Click on the "Yes" link to download each layer.

If the Download column still says "Register", you probably have not completed the registration process successfully. You will have to log back in if you have left the website or if your browser has been idle for too long. In either of these cases, just enter the e-mail address you registered with. You must have cookies enabled in order for the registration process to complete successfully.

6) 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 ERDAS LAN format. These formats are readable by the ESRI suite of products that include ArcExplorer, ArcView and Arc/Info. For info on these products, please go to www.esri.com. Other programs such as MapInfo and AutoCAD may also be able to read these formats.

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

7) What is the format for parcel numbers?

Parcel numbers are used to uniquely identify a piece of property. The parcel number, or APN, is a 12 digit number composed of 5 parts:

Example	Description	Lengthbr>
001	book number	3 digits
01	page number	2 digits
1	block number	1 digits
001	parcel number	3 digits
000	not currently used, set to 3 zeros	3 digits

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.